You're missing a big tuning CV carbs: the needle. On variable venturi carbs, the needle taper has a huge influence on fueling. The main jet size will have a big effect on high RPM WOT fueling, the needle everywhere else. If you are lean at high RPM WOT, you need a bigger main jet. If that then causes you to be rich at low RPM, you need a needle that has a larger diameter at its base. There are tons of different needle tapers available. Many needles are also adjustable. If you take the diaphragm off, you will see the needle is retained with a clip. Adjustable needles will have multiple grooves. Setting the needle deeper into the carb body (a higher groove) will lean out low and mid RPM. Pulling the needle up (lower groove) will enrichen low and mid RPM.
I didn't see this as I was typing my response, that needle is causing the problems he's having. both in height adjustment and I suspect the needle profile itself might be wrong for the motor. but he needs to get it much closer to optimal to tell for sure.
Motorcycle mechanic here: these carbs really really like to have a air inlet tube / velocity stack to make the airflow more directional , Just 2 inches can be enough with a pod filter, and this aids flow through the internal passages. also the low RPM air fuel ratio is adjusted by the mixture screw on the side, this can affect fueling upto 1/3 - 1/2 throttle (the CV slide not the butterfly)
Also the mounting boot on the engine is plenty sufficient to support the carb. I ran 4 carbs disconnected from the air box and ran a pair of siamese K&N filters with built in velocity stacks. Tuning was a bear but when I finally got it right the bike was insane.
As you can see in 4:16 there is a opening on the bottom of the intake side, when the air rushes throught the venturi, a part of it gets direktet up that hole, this is what opens your variable venturi. You could stick a gopro behind that carburator ans watch how it moves in relation to throttle padle position. it might not open more than 50%
depending on the setup. take a carbed harley, (later evo, early twin cam) with an actual keihin carb(this is a chicom knock off) it has no air boot or anything other than a metal flange to support and air filter.
Most motorcycles with everything right won't do anything but idle. Put on the air cleaner and watch that mixture go towards ride and take out that dip to rich that disappears as the RPM goes up. It's designed so as the airflow goes up, the air filter blocks the air more. The opposite of your graft is what the flow of the air boc. that needle you see inside the emulsion tube. I never change it just raise that needle up and down. Replacement carbs that needle has 5 places to raise or lower the needle. Just raise the needle and put on an Air filter. A piece of hose 2 feet long will sort of work 2 inch. Play with the diameter or just get the stock air box for a bike that uses that carbonator. Motorcycle junkyard will be your friend. You tell em what your using it for they likely give it to you. filter and all.. I do.
I did some tuning on a set of 4 CVK's. When these are setup right they are great. The trick is to start by setting the float height, then chosing the right main jet for max power. Then go on to the mid range tuning by raising or lowering the needle. Some needles have tiny c-clips that can be used to change needle height, fine tuning can be done by stacking tiny washers under those c-clips. In practice you will be driving/riding 'on the needle' 99% of the time so it's important to get it right. Finally you need to set the idle mixture and idle speed. When you're done you should have good idle, crisp throttle response off idle and good top end power. Setup like this you will also be able to get good milieage our of it as well.
The OEM CV needlee don't have the adjustment grooves (or at least my Keihin CV doesn't). I've reshaped an old needle before, but I found it really easy to mess up 😅
Use small washers to space the needle away from the slide diaphragm to richen the top end. 1st gen SV650s used to put 1-1.5 mm on the stock needles with a 152 main jet. I remember doing this mod after a full exhaust.
Tuning a CV carb is not just about the jet sizes, The needle shape is an important part of fine tuning. Also, washers can be used under the needle to raise it up if the needle does not have an E clip and adjustment grooves.
Definitely listen to people talking about needles, but also make sure you have an air box. On a bike, that air box keeps a large volume of relatively stable air for the engine to draw from. That gives consistency to the air part of the equation which is *far* less consistent with even a pod filter.
You need a bigger main jet! Main jet setting is for wot mixture. You need too drop the jet needle down or replace it with one with a fatter profile. The stock one should have a c clip and 5 positions to raise or lower. The emulsion tube is called a needle jet on that style of carb and usually isnt changed when tuning. You can also shorten or streach the spring in the slide to effect the venture sizing. Typically we shorten it when tuning for more fuel on modified bikes.
Most all older carbureted 4 wheelers/atvs use that enrichening valve setup versus an actual choke as well. My father argued this fact with me vehemently one day when I told him his wheeler didn’t have a choke but rather a small valve that lets more fuel in. He just refused to believe that the little lever that said choke wasn’t actually a choke until I had to literally pull the carb off and show him that there was only 1 butterfly in the carb and that was the throttle, and the “choke lever” he would move was just a cable that opened a fuel valve. 🤣
I don’t know about your carb but the SU type has a selection of different shaped needles for running. Different needle widths at different heights adjust the mixture for changing loads.. Great shows keep them coming.
I came here to say the same thing, use the jets to tune low load, and then file or turn down the needle at whatever load area to tune high load. (I have a classic mini background so used to messing with SU's .. as you are using this carburettor well out its intended use you may not be able to tune with just jetting.
Man, the needle controls the mixture between 1/4 and 3/4 throttle. You have different needle tapers and, sometimes, you can adjust needle height by changing the clip that fixes the needle to the sliding cilinder (we call that the "campana"). What I can tell is that you should lower the needle a coupla points, or change to another with less taper. Opening the idle mixture valve (on the side) can also change mixture richness. You can screw it til it tops, without exerting any kind of torque, then unscrew, say, one turn. Usually it is a turn and a half. That will make mixture leaner when the throttle starts opening, but will also affect idle.
Hey jimbo, lower the needle hight to remove the rich site of the curve and put a airfilter to make i run a bit richer and make the venturi work better. Keep up te good work!
That's where I would start. I'm not sure the emulsion tube is the way to go. I would drop needle first and then go up on the main jet. Mikuni and Keihin both have needle charts. I would go with a leaner needle before emulsion tube.
As a guy who's been working on bikes for 30 years I agree with the general consensus: Drop the needle, bigger main, play with the pilot and idle mixture screw to tweak the transition. Look at needle profiles as well. As someone who's put 2 48mm CV carbs from a VTR1000 on a Volvo B18 engine (It's LeMons, don't ask) the emulsion tube is one of the last things to change... Love the channel
I don't know why I love your channel so much. Normaly I am a big bore 2 stroke guy but there is no Sunday without Robot Cantina. Thank you for the Videos and greedings from Germany.
The air/fuel screw is where you adjust off idle and deceleration AFR. If it’s on the motor side, it adjusts fuel. If it’s on the air filter side, it adjusts air.
Hi Jimbo, I think the best way to correct the "lean/rich" curve you have is to try different postion of the needle, or buy one with a different curvature. But, as you say, if you can't get a rich mixture anymore with the main jet you need a new polverizer. Also, it's more easy to adjust carburation with an airbox...
The way you narrate your videos instead of talking in them is an odd choice for "cartube" but it really works. I'm not sure many other channels could pull it off, but it's kind of a staple at this point.
Short is ok. This is worth a second watch the view the very simple and easy to understand explanation of how a carburetor works. And the motor sounds really good.
I used to have a 65 Mustang with a Holley 600 on it heavily modified😮 I had it set so well that I could drive around on the primary two barrels all day without ever opening up the secondaries🎉 right around 30 miles to the gallon easy🎉 when I stepped on and it was like nitrous🎉 God I miss that car
Tuning small engine carburetors can be quite the art. I used to tune motorcycle carbs way back in the '70s, and it was always a delicate interplay of the main, pilot, needle and emulsion tubes. Of course we didn't have wide band ox sensors then, we had to make precision throttle runs and read the plug color, plus a lot of seat-of-the-pants measurements. I see people just throwing on a random carb without tuning and wondering why it doesn't run right without understanding why.
There have been so many times I've worked on stuff for friends, family members, neighbors, etc. Mowers, motorcycles, ATVs, basically anything with an engine. Most times they go to messing with stuff and mess the tuning all up and I have to put a new plug in and get them tuned right. Some of them can be a pain 😂 mad respect for you, I don't do performance stuff as far as smaller engines go, mostly just repairs.
I've essentially got that same pump/regulator/return line setup on my 350 S10. Works fine! I've experimented with a number of EFI systems and carbs on it over the last 15 or so years, and didn't want to drop the high pressure pump when I went from EFI (~55psi) to a carb (~7psi).
To the guys saying "bigger main jet" - the main alone doesn't control wide open fueling on these - its only allowed to if the slide pulls the needle far enough up. If this carb is set up for a bike its probably set up for a larger cylinder displacement, more compression and better high rpm head flow characteristics. That means its going to have a long, thin taper to the needle and a fairly stiff return spring over the slide. No watch what the engine is doing. Idle is fine so its in the ballpark for carb sizing as its set correctly without messing about with it. Then it goes rich at low load - suggesting as the throttle is open at lower rpm the fueling is still correct - though now not so thanks to the larger jet. This points to the carb behaving correctly and putting a normal sized main back in would correct this. Its then leaning out more and more as rpm rises. We know there is enough or too much fuel because it goes rich lower down so its a fuel command problem rather than one of availability. That means the slide is coming back down as rpm rises and the engine's flow dropes off. It only needs to come down a few mm and it will start closing that main which will then drop power and the cycle continues. The only way a bigger main fixes this is if its so large there is bypass of the needle. Thats not what you want on a cv because atomisation is then in the toilet and you're going to be on a good mixture but wondering why its got no response.
I had an SU carb on my turbo car in the early 80's - no EFI back then, really not so many turbos either! The tuner bloke mapped the power/AF Ratio across the band and honed the needle down - in a drill with sandpaper! Worked like a charm.
You need to lower the needle a couple of notches in order to lean off the part throttle mixture (raise the clip), you can then go bigger on the main jet in order to get the WOT right. The other thing to note is that you really need an air box on the back of the carb, the reason is that it is the slight vacuum in the airbox is 'transmitted' to the top of the carb via that hole at the back of the carb in order to lift the slide, at the moment I doubt your slide is lifting fully. This is the downside of a CV carb, there needs to be at least a small restriction in the intake side in order to create a vacuum at the back of the carb so that the diaphragm can lift the slide, a pure flat slide carb does not have this issue, of course on the plus side a CV carb will tolerate ham-fisted throttle abuse much better than a flat slide carb. Personally I would make sure that the slide is lifting fully at WOT (maybe via a camera), I'd then get the WOT mixture setup to around 12.8:1, I'd then use the needle to lean off the part throttle to around 14.5-15.5:1. If you can not get the part throttle mixture right then you are into using different needles which would get expensive!
Jimbo. In addition to what others have said about different needles. The Keihin CVK is also very sensitive to air filters/boxes, so do you jetting with one fitted otherwise you'll end up having to do it all again
Use small washers to space tge needle from its sest on the diaphragm slide. 1-1.5 mm sould make a noticable difference for mostly free. Or get a set of adjustable needles. The 1st gen SV650s did the washer mod after exhaust to fix a up top lean bog.
The variable carb thing looks interesting, i was wondering if that dip you see in air fuel has to do with how that venturi restriction opens up. Then you said emulsion tube and I sat back to learn more.. Looking forward to next week!
The CV carbs are phenomenal when they work correctly. I'm still in the early part of the episode but I'm hoping he uses a conventional air breather setup as these dont lend themselves well to "performance" pod type air filters. They need at least a tube between them and the filter to maintain a smooth vacuum reference. Folks take the air boxes off engines with these and then wonder why they run like garbage and have poor throttle response.
Been a long time since I fooled with a motorcycle carb, and actually, I never had jets to put in either car or motorcycle carbs. Always worked with what I had. I particularly liked the graphs you came up with. Now I know I need a lot of stuff if I'm going to tune a carb.
Needle will help but you also need some sort of velocity stack on there. Even if it's a stock air boot. The turbulence at the mouth of the carb will scree with the jetting. Going from a pod type filter on the back of the carb to a velocity stack with the pod filter on the end let me drop about 10 jet sizes and it ran much more consistently.
Once again I was left wanting more. Great work, the isssues will probably be solved by tomorrow and can't wait to see those numbers. Also RIP saturn. Died like a hero, teaching us very interesting stuff.
Thanks for another great video Jimbo. It may feel like you ended on a low note, but that's just part of experimentation and progress. Now you have more things to play with and make videos about. :) RIP Saturn. You were a great project car. You lived a good life.
5:37 I know it may seem like an obvious fix, but I love the solution you came up with. I don't think I would have come up with an idea like that if I was presented the same conundrum.
We use to put a fine screen between the carb and manifold to annimise the fuel ! I think your leaning out because of your fuel routing system , all you need is around 3 lbs of fuel pressure , so put the restrictor between the card and pump and dial it down without the return line ! I'd ditch the governor also !
Love this series.😎Good carb choice it's really hard to oversize a CV carb. You should make sure you have a 5 or 6 inches of tube on the intake side of the carb to contain the fuel reversion . If this is not done you might have some issues with the transition from idle and may need to almost double the size of the idle jet. Your main is too small. 0 to 1/4 throttle is idle jet, 1/4 to 3/4 throttle is the jet needle and needle jet, 3/4 to full is the main. Set the main first and work backwards. You shouldn't need to change the needle jet. More likely you just have to change the height of the jet needle. But set the main first.
Try stacking some washers under the needle in the cv portion of the carb. It will clear the main jet sooner. That with the smaller jet for off idle might help. Part of what we had to do to tune bikes after making changes to the intake and exhaust. Another person mentioned the air cleaner with a velocity stack. Cooler air might help.
You really know how to keep us hanging. lol I can't wait to see the solution to your carb tuning problem. I'm guessing that once you get the 670 running like you want that performance figures might not be that much different than the stock engine. A few performance enhancements might even yield faster times than the Renault's factory performance. Thanks for sharing. Phil
There's plenty of videos on youtube of lawnmower carbs on V8's gettting really good fuel economy. The higher displacement and small venturi make the air velocity very high and allows for good mixing.
The needle taper on the plunger is also part of the tuning on the carbs. since it changes the opening as it goes through the motions of the oceans. Not sure if you interested, but I have a carb that I think will work well for your application. Its from my Buick/Opel by isuzu, comes from 1.8L engine. has tiny primary and a little bigger vacuum secondaries. Though would need to make the intake adapter that would be at least 500cc in volume to keep it happy.
That fabricobbled carburetor support is going to force the float off it's seat and hammer the slide. Youre better off leaving just the boot. Also, if the needle doesnt have multiple grooves shim it up off the slide with a tiny washer or two.
Love your videos. You are a great teacher and shovel worthless knowledge into my skull in every video. I still think you should have put the worthless Satty on E-Bay or BAT and made a few bucks off it. But you do you, Dude.
Jimbo here's an experement to find out whats causing it. Jam the slide open and see what happens. If it fixes the lean issue you need a softer spring in the slide. Of course it will run like total garbage low down but its an easy way to figure out what to do next.
Great effort to gain the best performance on this cute little car. Jetting carburator sucks!!! I allready removed mine about 40 to 50 times to change jets or adjust floater...brrrrr. Cheers mate you do a great job. This little car is perfect for Farmer Markt shopping day in your village. Greetings from Barcelona...we Spanish love Renault.
As others have said, the needle needs adjusting for in-between area. In fact IIRC, the sequence is: Slow jet, slide cutaway, jet needle, main jet. I am actually sceptical this single carb will work at all. You don't need carb sized for 670cc engine, since only one cylinder is sucking at a time. 32mm would be fine for a 670cc single, not a V-twin. Also, the intake pulses are not equally spaced, so you will never get both cylinders running the same. That is the reason the engine came with two carbs. It would have been cheaper to sell it with a single larger carb but they did not do this because it doesn't work as well.
First of all: I am by no means an expert in this style of carbs but if I understand correctly the negative pressure opens the venturi. If this is true, you could also try to get a valve in between the low pressure side and the system which would make the venturi adjustable. Just an idea..... Another idea: In theory it should be possible to rig up an injection system to counteract the lean mixture like Kugelfischer did way back when.
Jet back down and raise the needle or get a lighter weight spring in there. Its going rich because the main is too large then its leaning out because as the rpm rises the engine isn't making enough vaccuum to keep the slide fully open. The emulsion tube is just helping atomisation - it has no real say on overall fueling until you're really far off what the carb is set up for.
Excellent work as always, Jimbo. Always look forward to your Sunday videos. Sure sucks to see 128k views and only 1k likes though. You deserve so much more for all the work that you put into your videos. Thanks for always keeping me entertained!
Where the carb adaptor block meets the engine manifold is an issue. The two holes would benefit greatly from being modified to provide smoother air flow into the manifold. As it is, the abrupt hole will be doing bad things for the flow. There isn't enough room for a pair of stub-stacks, so out with the grinder to add a smooth transition into the port. It'd benefit to remove the material between the ports to provide a smooth, knife-edge bridge between the two. Imagine a fat infinity sign. A 45 degree cut into the ports to begin with, then smooth the edge and round it out to provide a sort of 'sunken' stack into the port, then remove material between the ports to narrow that ugly bridge and provide smooth flow. The idle circuit adds a lot to the throttle up to 1/3~1/2 throttle on a CV. You may be able to lean that out. Many CV carbs give you the option of different needle profiles too. If you can't get a needle chart, try shimming the needle within the throttle slide. You can now have a leaner idle, then as the slide rises, the richer part of the main needle is used earlier. Hope that helps.
I like how you make brackets that mount to exsiting hardware. That's how I like to do it, but my friends just like to hobble stuff together that causes problems down the road.
There is so much aftermarket support for these engines. We would all love to see you do valve springs and the biggest CAM you can fit. Or they also have race ready heads for these as well. Heads are best but cams are great as well or if wanna go cheaspeat and easiest then ratio rockers are good as well
Years and years ago when I was racing an MGB with dual SU carbs, we would be messing with needle profiles at this point. It was pretty much guess work. I wish we had a wideband gauge back then.
To add to what everyone else is saying about the various aspects of fuel contributions - mikuni has published a neat little chart on how the idle, pilot, needle and main jet contribute fuel.
A big motorcycle airbox on the carb would also help the total drag coefficient of the vehicle by sitting behind the rear glass, creating a stepped tail profile.
lower the needle inside the slide, that would require you to raise the snap ring to a higher position on the needle. I'd go two for as rich as you were. Awesome content, and good luck with your project.
Pilot controls fuel for idle through 1/4 throttle. From there its needle jet and main jet. If you need more fuel at wot, you will have to move the clip on the needle. Only after you run out of notches, do you change the venturi tube, but even then, you are replacing the needle to match. Make your pilot a size smaller, and raise the needle a notch.
Most of the engines I tinker with wouldn’t like that sharp 90° turn down just past the carb. Turbulence would cause the fuel to come out of suspension and form droplets on the surface making tuning erratic and difficult. A curved transition might be more stable across the RPM range.
I gotta go to work so I'll finish the vid later but you need two of these carbs. I can explain why later but if staying CV you should find yourself loving that size x2 or one 45mm flat slide like an hsr would be great too. Also 2 32mm CV carbs is still choking this thing. 670cc ÷2 335cc and on motorcycles we normally see gains going bigger than stock and stock for honda would be closer to 36mm although I'll admit the 670 has trash ports and heads and the runner length matters dont make it too short if you do go with two
Some of us were brought up on SU and Stromberg carbs and appreciate how how clever they are, but I didn't appreciate that VV carbs were still used on bikes.
You can tune mid rpm range by raising or lowering the needle and also changing the stiffness of the spring that holds the slide down it's a very adjustable carb
Don't forget about the CV needle that goes into the emulsion tube and uncovers holes as it's moving up. Hopefully yours is adjustable. Of course I'm sure it's already in the works for the next video ;-)
Loving your stuff, very interesting solution to the two bolt occupying the same place in the space time continuum. Learn something new every time I watch, looking forward to next week!!
You had a first gen Honda Insight so you know about how the special lean burn mode works on the manual transmission cars . It goes up to 22:1 and the fuel economy goes up ( power goes down). So you need a two mode car rich for 0-60 and super lean for cruise. Wiki quote A lean burn mode is a way to reduce throttling losses. An engine in a typical vehicle is sized for providing the power desired for acceleration, but must operate well below that point in normal steady-speed operation. Ordinarily, the power is cut by partially closing a throttle. However, the extra work done in pumping air through the throttle reduces efficiency. If the fuel/air ratio is reduced, then lower power can be achieved with the throttle closer to fully open, and the efficiency during normal driving (below the maximum torque capability of the engine) can be higher. The engines designed for lean-burning can employ higher compression ratios and thus provide better performance, efficient fuel use and low exhaust hydrocarbon emissions than those found in conventional gasoline engines. Ultra lean mixtures with very high air-fuel ratios can only be achieved by direct injection engines. The main drawback of lean-burning is that a complex catalytic converter system is required to reduce NOx emissions. Lean-burn engines do not work well with modern 3-way catalytic converter-which require a pollutant balance at the exhaust port so they can carry out oxidation and reduction reactions-so most modern engines tend to cruise and coastdown at or near the stoichiometric point. “
I got that same electric fuel pump for my mower due to the same issues on one of my garden tractors. .. I was also planning to add a return line. Wasn't planning on the 'dead head' regulator, was maybe going to put a valve on... But guess I'll add that to the list.
On a motorcycle carb the main jet is responsible mainly for 3/4 to fully open the pilot up to 1/8th throttle , the midrange is usually controlled by the slide needle position, or you can alter the ones in the mouth of the carb the main air jet and the pilot air jet, hope you find this of some help... pilot air jet looks likely, it causes problems on the transition from the pilot to the main system
It would be fantastic to see you bring the Renault to McPherson to show off to the Automotive Restoration students at McPherson college. I’m sure we’d all love to pick your brain on everything having to do with this car and we have drive ins and car show’s pretty often
I say that when the engine starts getting more heavily modded for power and you have more money to spend, ditch the Keihin and get a SmartCarb. They don't use jets and instead rely on their patented metering rod in order to mix the air and fuel, and use integrated, internal venting directly at the venturi opening which communicates both atmospheric pressure and manifold absolute pressure (MAP) into the float bowl, driving fuel flow proportional to both air density and velocity in real-time. In layman's terms, it's a heck of a lot easier to tune and won't need adjustments for atmospheric changes because it does all the work itself.
The needle meters the fuel at part throttle. It's not gonna be easy, but you need to figure that out. These carbs can be pretty complicated to tune if you're using it in an application it wasn't designed for. It sounds like you need a bigger main jet and a fatter needle.
I think you're headed in the right direction with the emulsion tubes. The bigger your air bleeds in a fuel circuit, the less signal that it will get, leaning out the mixture at lower rpms. You would also need a bigger jet to make up for the added air that is being introduced to the circuit. Also, I'm not too familiar with the CVK carbs, but the idle circuit often has impact on low speed and part throttle mixtures.
You're missing a big tuning CV carbs: the needle. On variable venturi carbs, the needle taper has a huge influence on fueling. The main jet size will have a big effect on high RPM WOT fueling, the needle everywhere else.
If you are lean at high RPM WOT, you need a bigger main jet. If that then causes you to be rich at low RPM, you need a needle that has a larger diameter at its base. There are tons of different needle tapers available.
Many needles are also adjustable. If you take the diaphragm off, you will see the needle is retained with a clip. Adjustable needles will have multiple grooves. Setting the needle deeper into the carb body (a higher groove) will lean out low and mid RPM. Pulling the needle up (lower groove) will enrichen low and mid RPM.
Came here to say this. Main jet it for WOT, then use different needle profiles and heights to tune the part throttle.
I just learned something here! Excellent post!
Same.
@@Stevemax07it's good info! It translates across some other types of carbs too, bits of it.
I didn't see this as I was typing my response, that needle is causing the problems he's having. both in height adjustment and I suspect the needle profile itself might be wrong for the motor. but he needs to get it much closer to optimal to tell for sure.
Motorcycle mechanic here: these carbs really really like to have a air inlet tube / velocity stack to make the airflow more directional , Just 2 inches can be enough with a pod filter, and this aids flow through the internal passages. also the low RPM air fuel ratio is adjusted by the mixture screw on the side, this can affect fueling upto 1/3 - 1/2 throttle (the CV slide not the butterfly)
Also the mounting boot on the engine is plenty sufficient to support the carb. I ran 4 carbs disconnected from the air box and ran a pair of siamese K&N filters with built in velocity stacks. Tuning was a bear but when I finally got it right the bike was insane.
As you can see in 4:16 there is a opening on the bottom of the intake side, when the air rushes throught the venturi, a part of it gets direktet up that hole, this is what opens your variable venturi.
You could stick a gopro behind that carburator ans watch how it moves in relation to throttle padle position. it might not open more than 50%
depending on the setup. take a carbed harley, (later evo, early twin cam) with an actual keihin carb(this is a chicom knock off) it has no air boot or anything other than a metal flange to support and air filter.
Most motorcycles with everything right won't do anything but idle. Put on the air cleaner and watch that mixture go towards ride and take out that dip to rich that disappears as the RPM goes up. It's designed so as the airflow goes up, the air filter blocks the air more. The opposite of your graft is what the flow of the air boc. that needle you see inside the emulsion tube. I never change it just raise that needle up and down. Replacement carbs that needle has 5 places to raise or lower the needle. Just raise the needle and put on an Air filter. A piece of hose 2 feet long will sort of work 2 inch. Play with the diameter or just get the stock air box for a bike that uses that carbonator. Motorcycle junkyard will be your friend. You tell em what your using it for they likely give it to you. filter and all.. I do.
I did some tuning on a set of 4 CVK's. When these are setup right they are great. The trick is to start by setting the float height, then chosing the right main jet for max power.
Then go on to the mid range tuning by raising or lowering the needle. Some needles have tiny c-clips that can be used to change needle height, fine tuning can be done by stacking tiny washers under those c-clips. In practice you will be driving/riding 'on the needle' 99% of the time so it's important to get it right.
Finally you need to set the idle mixture and idle speed.
When you're done you should have good idle, crisp throttle response off idle and good top end power.
Setup like this you will also be able to get good milieage our of it as well.
Needs a fatter needle on the end , generally they are twin tapered. Drop the pilot one size .
Or maybe just drop the needle, the majority of motorcycle carbs have the height on the needle adjustable.
The OEM CV needlee don't have the adjustment grooves (or at least my Keihin CV doesn't). I've reshaped an old needle before, but I found it really easy to mess up 😅
@@Lucaspc99drop and polish the top with a drill an 600
Use small washers to space the needle away from the slide diaphragm to richen the top end. 1st gen SV650s used to put 1-1.5 mm on the stock needles with a 152 main jet. I remember doing this mod after a full exhaust.
Then it goes waayy leaner, this could be just fixed by lifting the needle with the clip
Needle tuning is the most important on that style carburetor. You're overlooking a huge step. The main jet only does WOT
Main jet is more important on 3/4-full throttle than the needle
@@Hnkka I am one of about 1000 posts agreeing that needle adjust is the prob. Make sure you reply to every one of those.
Why so offended?@@mastermnd22
Tuning a CV carb is not just about the jet sizes, The needle shape is an important part of fine tuning. Also, washers can be used under the needle to raise it up if the needle does not have an E clip and adjustment grooves.
Definitely listen to people talking about needles, but also make sure you have an air box. On a bike, that air box keeps a large volume of relatively stable air for the engine to draw from. That gives consistency to the air part of the equation which is *far* less consistent with even a pod filter.
You need a bigger main jet! Main jet setting is for wot mixture. You need too drop the jet needle down or replace it with one with a fatter profile. The stock one should have a c clip and 5 positions to raise or lower. The emulsion tube is called a needle jet on that style of carb and usually isnt changed when tuning. You can also shorten or streach the spring in the slide to effect the venture sizing. Typically we shorten it when tuning for more fuel on modified bikes.
Most all older carbureted 4 wheelers/atvs use that enrichening valve setup versus an actual choke as well. My father argued this fact with me vehemently one day when I told him his wheeler didn’t have a choke but rather a small valve that lets more fuel in. He just refused to believe that the little lever that said choke wasn’t actually a choke until I had to literally pull the carb off and show him that there was only 1 butterfly in the carb and that was the throttle, and the “choke lever” he would move was just a cable that opened a fuel valve. 🤣
My favorite before church show to watch! I love how well thought out your projects are and how well they work.
I don’t know about your carb but the SU type has a selection of different shaped needles for running. Different needle widths at different heights adjust the mixture for changing loads.. Great shows keep them coming.
I came here to say the same thing, use the jets to tune low load, and then file or turn down the needle at whatever load area to tune high load. (I have a classic mini background so used to messing with SU's .. as you are using this carburettor well out its intended use you may not be able to tune with just jetting.
you're an excellent narrator and a good script writer. concise, funny, and surprising
thanks!
Man, the needle controls the mixture between 1/4 and 3/4 throttle. You have different needle tapers and, sometimes, you can adjust needle height by changing the clip that fixes the needle to the sliding cilinder (we call that the "campana"). What I can tell is that you should lower the needle a coupla points, or change to another with less taper. Opening the idle mixture valve (on the side) can also change mixture richness. You can screw it til it tops, without exerting any kind of torque, then unscrew, say, one turn. Usually it is a turn and a half. That will make mixture leaner when the throttle starts opening, but will also affect idle.
Hey jimbo, lower the needle hight to remove the rich site of the curve and put a airfilter to make i run a bit richer and make the venturi work better. Keep up te good work!
That's where I would start. I'm not sure the emulsion tube is the way to go. I would drop needle first and then go up on the main jet.
Mikuni and Keihin both have needle charts. I would go with a leaner needle before emulsion tube.
As a guy who's been working on bikes for 30 years I agree with the general consensus: Drop the needle, bigger main, play with the pilot and idle mixture screw to tweak the transition. Look at needle profiles as well. As someone who's put 2 48mm CV carbs from a VTR1000 on a Volvo B18 engine (It's LeMons, don't ask) the emulsion tube is one of the last things to change... Love the channel
I wanna know about the LeMons car lol
Well said.
I don't know why I love your channel so much. Normaly I am a big bore 2 stroke guy but there is no Sunday without Robot Cantina.
Thank you for the Videos and greedings from Germany.
The air/fuel screw is where you adjust off idle and deceleration AFR. If it’s on the motor side, it adjusts fuel. If it’s on the air filter side, it adjusts air.
Hi Jimbo, I think the best way to correct the "lean/rich" curve you have is to try different postion of the needle, or buy one with a different curvature. But, as you say, if you can't get a rich mixture anymore with the main jet you need a new polverizer. Also, it's more easy to adjust carburation with an airbox...
Loving the Renault series
I agree with this statement
The way you narrate your videos instead of talking in them is an odd choice for "cartube" but it really works. I'm not sure many other channels could pull it off, but it's kind of a staple at this point.
Good bye dear Saturn.
Your contributions to Jimbo's mad science church will not go unnoticed and unrecognized.
You'll be missed and remembered. ✊😢
“…acceleration in a hurry”
What a quote! 😂😂😂
a tear rolled down my cheek watching the Saturn get towed away.
Short is ok. This is worth a second watch the view the very simple and easy to understand explanation of how a carburetor works. And the motor sounds really good.
I used to have a 65 Mustang with a Holley 600 on it heavily modified😮 I had it set so well that I could drive around on the primary two barrels all day without ever opening up the secondaries🎉 right around 30 miles to the gallon easy🎉 when I stepped on and it was like nitrous🎉 God I miss that car
Tuning small engine carburetors can be quite the art. I used to tune motorcycle carbs way back in the '70s, and it was always a delicate interplay of the main, pilot, needle and emulsion tubes. Of course we didn't have wide band ox sensors then, we had to make precision throttle runs and read the plug color, plus a lot of seat-of-the-pants measurements. I see people just throwing on a random carb without tuning and wondering why it doesn't run right without understanding why.
There have been so many times I've worked on stuff for friends, family members, neighbors, etc. Mowers, motorcycles, ATVs, basically anything with an engine. Most times they go to messing with stuff and mess the tuning all up and I have to put a new plug in and get them tuned right. Some of them can be a pain 😂 mad respect for you, I don't do performance stuff as far as smaller engines go, mostly just repairs.
I've essentially got that same pump/regulator/return line setup on my 350 S10. Works fine! I've experimented with a number of EFI systems and carbs on it over the last 15 or so years, and didn't want to drop the high pressure pump when I went from EFI (~55psi) to a carb (~7psi).
To the guys saying "bigger main jet" - the main alone doesn't control wide open fueling on these - its only allowed to if the slide pulls the needle far enough up.
If this carb is set up for a bike its probably set up for a larger cylinder displacement, more compression and better high rpm head flow characteristics.
That means its going to have a long, thin taper to the needle and a fairly stiff return spring over the slide.
No watch what the engine is doing.
Idle is fine so its in the ballpark for carb sizing as its set correctly without messing about with it.
Then it goes rich at low load - suggesting as the throttle is open at lower rpm the fueling is still correct - though now not so thanks to the larger jet. This points to the carb behaving correctly and putting a normal sized main back in would correct this.
Its then leaning out more and more as rpm rises.
We know there is enough or too much fuel because it goes rich lower down so its a fuel command problem rather than one of availability.
That means the slide is coming back down as rpm rises and the engine's flow dropes off.
It only needs to come down a few mm and it will start closing that main which will then drop power and the cycle continues.
The only way a bigger main fixes this is if its so large there is bypass of the needle. Thats not what you want on a cv because atomisation is then in the toilet and you're going to be on a good mixture but wondering why its got no response.
I had an SU carb on my turbo car in the early 80's - no EFI back then, really not so many turbos either! The tuner bloke mapped the power/AF Ratio across the band and honed the needle down - in a drill with sandpaper! Worked like a charm.
Before we get to the end of this, can we at least contemplate a super charger pls
You need to lower the needle a couple of notches in order to lean off the part throttle mixture (raise the clip), you can then go bigger on the main jet in order to get the WOT right. The other thing to note is that you really need an air box on the back of the carb, the reason is that it is the slight vacuum in the airbox is 'transmitted' to the top of the carb via that hole at the back of the carb in order to lift the slide, at the moment I doubt your slide is lifting fully. This is the downside of a CV carb, there needs to be at least a small restriction in the intake side in order to create a vacuum at the back of the carb so that the diaphragm can lift the slide, a pure flat slide carb does not have this issue, of course on the plus side a CV carb will tolerate ham-fisted throttle abuse much better than a flat slide carb. Personally I would make sure that the slide is lifting fully at WOT (maybe via a camera), I'd then get the WOT mixture setup to around 12.8:1, I'd then use the needle to lean off the part throttle to around 14.5-15.5:1. If you can not get the part throttle mixture right then you are into using different needles which would get expensive!
Jimbo. In addition to what others have said about different needles. The Keihin CVK is also very sensitive to air filters/boxes, so do you jetting with one fitted otherwise you'll end up having to do it all again
Thanks for all your experiments Jimbo.. lots of subtle and not so subtle lessons to take away.
You need an airbox with a cv carb for it to work properly
Use small washers to space tge needle from its sest on the diaphragm slide. 1-1.5 mm sould make a noticable difference for mostly free. Or get a set of adjustable needles. The 1st gen SV650s did the washer mod after exhaust to fix a up top lean bog.
The variable carb thing looks interesting, i was wondering if that dip you see in air fuel has to do with how that venturi restriction opens up. Then you said emulsion tube and I sat back to learn more.. Looking forward to next week!
The CV carbs are phenomenal when they work correctly. I'm still in the early part of the episode but I'm hoping he uses a conventional air breather setup as these dont lend themselves well to "performance" pod type air filters. They need at least a tube between them and the filter to maintain a smooth vacuum reference. Folks take the air boxes off engines with these and then wonder why they run like garbage and have poor throttle response.
Oh man!!! Now that’s what you call a cliffhanger. Can’t wait for the next Sunday.
I just love your sense of humor and how you teach us about what's going on so can fully understand your struggles
Been a long time since I fooled with a motorcycle carb, and actually, I never had jets to put in either car or motorcycle carbs. Always worked with what I had. I particularly liked the graphs you came up with. Now I know I need a lot of stuff if I'm going to tune a carb.
Needle will help but you also need some sort of velocity stack on there. Even if it's a stock air boot. The turbulence at the mouth of the carb will scree with the jetting. Going from a pod type filter on the back of the carb to a velocity stack with the pod filter on the end let me drop about 10 jet sizes and it ran much more consistently.
Once again I was left wanting more. Great work, the isssues will probably be solved by tomorrow and can't wait to see those numbers.
Also RIP saturn. Died like a hero, teaching us very interesting stuff.
Thanks for another great video Jimbo. It may feel like you ended on a low note, but that's just part of experimentation and progress. Now you have more things to play with and make videos about. :)
RIP Saturn. You were a great project car. You lived a good life.
Great job explaining how the carb and venturi work BTW 👏
Jesus Christ Jimbo, that intake manifold fix was just genius. Hope you can solve the issues with the tune soon. Greetings from Spain!
5:37 I know it may seem like an obvious fix, but I love the solution you came up with. I don't think I would have come up with an idea like that if I was presented the same conundrum.
Your crickets are making my crickets cricket.
We use to put a fine screen between the carb and manifold to annimise the fuel ! I think your leaning out because of your fuel routing system , all you need is around 3 lbs of fuel pressure , so put the restrictor between the card and pump and dial it down without the return line ! I'd ditch the governor also !
Plenty of comments have noted the importance of the needle and airbox / filter on a CV. I'm just here to +1 :)
I never fail to learn something while watching your videos, thanks Jimbo.
Love this channel! I too find nitrile gloves work best with holes in them!
Love this series.😎Good carb choice it's really hard to oversize a CV carb. You should make sure you have a 5 or 6 inches of tube on the intake side of the carb to contain the fuel reversion . If this is not done you might have some issues with the transition from idle and may need to almost double the size of the idle jet. Your main is too small. 0 to 1/4 throttle is idle jet, 1/4 to 3/4 throttle is the jet needle and needle jet, 3/4 to full is the main. Set the main first and work backwards. You shouldn't need to change the needle jet. More likely you just have to change the height of the jet needle. But set the main first.
Try stacking some washers under the needle in the cv portion of the carb. It will clear the main jet sooner. That with the smaller jet for off idle might help.
Part of what we had to do to tune bikes after making changes to the intake and exhaust.
Another person mentioned the air cleaner with a velocity stack. Cooler air might help.
Nicely illustrated work on the carb explanations.
Recycle in peace little Saturn.
The main jet needle insertion depth is another area the controls the A/F Ratio.
You really know how to keep us hanging. lol I can't wait to see the solution to your carb tuning problem. I'm guessing that once you get the 670 running like you want that performance figures might not be that much different than the stock engine. A few performance enhancements might even yield faster times than the Renault's factory performance. Thanks for sharing. Phil
There's plenty of videos on youtube of lawnmower carbs on V8's gettting really good fuel economy. The higher displacement and small venturi make the air velocity very high and allows for good mixing.
The needle taper on the plunger is also part of the tuning on the carbs. since it changes the opening as it goes through the motions of the oceans.
Not sure if you interested, but I have a carb that I think will work well for your application. Its from my Buick/Opel by isuzu, comes from 1.8L engine. has tiny primary and a little bigger vacuum secondaries. Though would need to make the intake adapter that would be at least 500cc in volume to keep it happy.
Thanks for the offer! I have my hands full at the moment with this carburator.
That fabricobbled carburetor support is going to force the float off it's seat and hammer the slide.
Youre better off leaving just the boot.
Also, if the needle doesnt have multiple grooves shim it up off the slide with a tiny washer or two.
Bye bye Saturn. Good job on a ton of work, wonder what the results will be next week. Keep up the interesting videos.
I definitely think down a notch on needle for part throttle and possibly up a size or two on main jets.
Some of the best edutainment on CZcams! 🤘🏻💀🤘🏻
Vroom vroom! Back at it again!
Love your videos. You are a great teacher and shovel worthless knowledge into my skull in every video. I still think you should have put the worthless Satty on E-Bay or BAT and made a few bucks off it. But you do you, Dude.
Jimbo here's an experement to find out whats causing it. Jam the slide open and see what happens.
If it fixes the lean issue you need a softer spring in the slide.
Of course it will run like total garbage low down but its an easy way to figure out what to do next.
Thanks for keeping your upload schedule. Nice to wake up and watch this with my Sunday morning coffee.
Love the new carb... You'll be able to get er dialed in!
Much easier to access than a 4 cylinder motorcycle!
Great effort to gain the best performance on this cute little car. Jetting carburator sucks!!! I allready removed mine about 40 to 50 times to change jets or adjust floater...brrrrr. Cheers mate you do a great job. This little car is perfect for Farmer Markt shopping day in your village. Greetings from Barcelona...we Spanish love Renault.
Greetings from Kansas USA! This little car is a blast to drive around town.
As others have said, the needle needs adjusting for in-between area. In fact IIRC, the sequence is: Slow jet, slide cutaway, jet needle, main jet. I am actually sceptical this single carb will work at all. You don't need carb sized for 670cc engine, since only one cylinder is sucking at a time. 32mm would be fine for a 670cc single, not a V-twin. Also, the intake pulses are not equally spaced, so you will never get both cylinders running the same. That is the reason the engine came with two carbs. It would have been cheaper to sell it with a single larger carb but they did not do this because it doesn't work as well.
Yup, you are right
First of all: I am by no means an expert in this style of carbs but if I understand correctly the negative pressure opens the venturi. If this is true, you could also try to get a valve in between the low pressure side and the system which would make the venturi adjustable.
Just an idea.....
Another idea:
In theory it should be possible to rig up an injection system to counteract the lean mixture like Kugelfischer did way back when.
I felt like saluting, as the Saturn was loaded up.
Will go down in CZcams lore.
Jet back down and raise the needle or get a lighter weight spring in there. Its going rich because the main is too large then its leaning out because as the rpm rises the engine isn't making enough vaccuum to keep the slide fully open.
The emulsion tube is just helping atomisation - it has no real say on overall fueling until you're really far off what the carb is set up for.
Excellent work as always, Jimbo. Always look forward to your Sunday videos. Sure sucks to see 128k views and only 1k likes though. You deserve so much more for all the work that you put into your videos. Thanks for always keeping me entertained!
Where the carb adaptor block meets the engine manifold is an issue. The two holes would benefit greatly from being modified to provide smoother air flow into the manifold. As it is, the abrupt hole will be doing bad things for the flow.
There isn't enough room for a pair of stub-stacks, so out with the grinder to add a smooth transition into the port.
It'd benefit to remove the material between the ports to provide a smooth, knife-edge bridge between the two. Imagine a fat infinity sign.
A 45 degree cut into the ports to begin with, then smooth the edge and round it out to provide a sort of 'sunken' stack into the port, then remove material between the ports to narrow that ugly bridge and provide smooth flow.
The idle circuit adds a lot to the throttle up to 1/3~1/2 throttle on a CV. You may be able to lean that out.
Many CV carbs give you the option of different needle profiles too. If you can't get a needle chart, try shimming the needle within the throttle slide. You can now have a leaner idle, then as the slide rises, the richer part of the main needle is used earlier.
Hope that helps.
I like how you make brackets that mount to exsiting hardware. That's how I like to do it, but my friends just like to hobble stuff together that causes problems down the road.
Great work professor!
There is so much aftermarket support for these engines. We would all love to see you do valve springs and the biggest CAM you can fit. Or they also have race ready heads for these as well. Heads are best but cams are great as well or if wanna go cheaspeat and easiest then ratio rockers are good as well
Years and years ago when I was racing an MGB with dual SU carbs, we would be messing with needle profiles at this point. It was pretty much guess work. I wish we had a wideband gauge back then.
To add to what everyone else is saying about the various aspects of fuel contributions - mikuni has published a neat little chart on how the idle, pilot, needle and main jet contribute fuel.
A big motorcycle airbox on the carb would also help the total drag coefficient of the vehicle by sitting behind the rear glass, creating a stepped tail profile.
lower the needle inside the slide, that would require you to raise the snap ring to a higher position on the needle. I'd go two for as rich as you were. Awesome content, and good luck with your project.
The needle is probably your issue, not the emulsion tube. the needles vary in taper and clip positions. :)
Rest In Pieces, Saturn.
Pilot controls fuel for idle through 1/4 throttle. From there its needle jet and main jet. If you need more fuel at wot, you will have to move the clip on the needle. Only after you run out of notches, do you change the venturi tube, but even then, you are replacing the needle to match.
Make your pilot a size smaller, and raise the needle a notch.
Most of the engines I tinker with wouldn’t like that sharp 90° turn down just past the carb. Turbulence would cause the fuel to come out of suspension and form droplets on the surface making tuning erratic and difficult. A curved transition might be more stable across the RPM range.
I gotta go to work so I'll finish the vid later but you need two of these carbs. I can explain why later but if staying CV you should find yourself loving that size x2 or one 45mm flat slide like an hsr would be great too. Also 2 32mm CV carbs is still choking this thing. 670cc ÷2 335cc and on motorcycles we normally see gains going bigger than stock and stock for honda would be closer to 36mm although I'll admit the 670 has trash ports and heads and the runner length matters dont make it too short if you do go with two
Some of us were brought up on SU and Stromberg carbs and appreciate how how clever they are, but I didn't appreciate that VV carbs were still used on bikes.
What a very good explanation about the variable veturi and why it matters to the small engines, thanks😊
You can tune mid rpm range by raising or lowering the needle and also changing the stiffness of the spring that holds the slide down it's a very adjustable carb
Don't forget about the CV needle that goes into the emulsion tube and uncovers holes as it's moving up. Hopefully yours is adjustable. Of course I'm sure it's already in the works for the next video ;-)
I was expecting the gnomes to return to wave goodbye to the Saturn, and maybe even go inside and leave with it.
Loving your stuff, very interesting solution to the two bolt occupying the same place in the space time continuum. Learn something new every time I watch, looking forward to next week!!
You had a first gen Honda Insight so you know about how the special lean burn mode works on the manual transmission cars . It goes up to 22:1 and the fuel economy goes up ( power goes down).
So you need a two mode car rich for 0-60 and super lean for cruise.
Wiki quote
A lean burn mode is a way to reduce throttling losses. An engine in a typical vehicle is sized for providing the power desired for acceleration, but must operate well below that point in normal steady-speed operation. Ordinarily, the power is cut by partially closing a throttle. However, the extra work done in pumping air through the throttle reduces efficiency. If the fuel/air ratio is reduced, then lower power can be achieved with the throttle closer to fully open, and the efficiency during normal driving (below the maximum torque capability of the engine) can be higher.
The engines designed for lean-burning can employ higher compression ratios and thus provide better performance, efficient fuel use and low exhaust hydrocarbon emissions than those found in conventional gasoline engines. Ultra lean mixtures with very high air-fuel ratios can only be achieved by direct injection engines.
The main drawback of lean-burning is that a complex catalytic converter system is required to reduce NOx emissions. Lean-burn engines do not work well with modern 3-way catalytic converter-which require a pollutant balance at the exhaust port so they can carry out oxidation and reduction reactions-so most modern engines tend to cruise and coastdown at or near the stoichiometric point.
“
I got that same electric fuel pump for my mower due to the same issues on one of my garden tractors. .. I was also planning to add a return line. Wasn't planning on the 'dead head' regulator, was maybe going to put a valve on... But guess I'll add that to the list.
On a motorcycle carb the main jet is responsible mainly for 3/4 to fully open the pilot up to 1/8th throttle , the midrange is usually controlled by the slide needle position, or you can alter the ones in the mouth of the carb the main air jet and the pilot air jet, hope you find this of some help... pilot air jet looks likely, it causes problems on the transition from the pilot to the main system
It would be fantastic to see you bring the Renault to McPherson to show off to the Automotive Restoration students at McPherson college. I’m sure we’d all love to pick your brain on everything having to do with this car and we have drive ins and car show’s pretty often
I say that when the engine starts getting more heavily modded for power and you have more money to spend, ditch the Keihin and get a SmartCarb. They don't use jets and instead rely on their patented metering rod in order to mix the air and fuel, and use integrated, internal venting directly at the venturi opening which communicates both atmospheric pressure and manifold absolute pressure (MAP) into the float bowl, driving fuel flow proportional to both air density and velocity in real-time. In layman's terms, it's a heck of a lot easier to tune and won't need adjustments for atmospheric changes because it does all the work itself.
RIP Saturn. Can't wait to see what the Kabota goes into next!
I LOVE this channel, the work you're doing. I have learned so much (and I already knew a bit)! Please keep up the most excellent work!!
Once again sitting here with my coffee and Robot Cantina.
The needle meters the fuel at part throttle. It's not gonna be easy, but you need to figure that out. These carbs can be pretty complicated to tune if you're using it in an application it wasn't designed for. It sounds like you need a bigger main jet and a fatter needle.
I think you're headed in the right direction with the emulsion tubes. The bigger your air bleeds in a fuel circuit, the less signal that it will get, leaning out the mixture at lower rpms. You would also need a bigger jet to make up for the added air that is being introduced to the circuit. Also, I'm not too familiar with the CVK carbs, but the idle circuit often has impact on low speed and part throttle mixtures.