Using Carburetor Synchronizer Gauges To Improve Honda CB750 Cafe Racer Motorcycle Running | 27
Vložit
- čas přidán 10. 12. 2023
- Using Carburetor Synchronizer Gauges To Improve Honda CB750 Cafe Racer Motorcycle Running | 27
In this video I am still trying to improve the running of my CB750 SOHC Cafe Racer project bike following major work on the engine. It isn’t running on all cylinders consistently but I think if the carbs are synchronized it will solve the problem.
Things that I used:
Carburetor Synchronizer Gauges: ebay.us/g2cm5F
Motorcycle Auxiliary Fuel Tank: ebay.us/7aeVud
After giving the carburettors another clean in the last episode I am sure that all cylinders run, but that number 2 doesn’t run as well as the others. I only know this by measuring the temperature of the exhaust where it comes out of the cylinder. For cylinder 2 it is the same as the others on choke, but gets cooler when the choke is off, showing it isn’t burning as much fuel as the others. As I commented in the previous video I suspect it is running well enough so that it wouldn’t be obvious when riding the bike. Carburettors on motorcycles often go out of synchronisation, with various symptoms, including; loss of power; poor idling; flat spots; stutters. Frequently these changes happen slowly and the rider adjusts around them, often they are not bad enough to stop you riding the bike.
Synchronisation of the carburettors is the way to make each cylinder give the same power all the way through the rev range. Most people think that it is getting the slides or butterflies to open at the same time, but really it is to get the amount of fuel and air going into the engine equal. You measure this with a vacuum gauge so that each cylinder is pulling the same amount of fuel and air.
Each cylinder will be slightly different, either the inlet tract might be slightly longer or a different shape the actual tolerances on each component might be a little different, but the power each produces is simply based on how much fuel and air it burns, so getting this the same for all cylinders will make it run much better.
Obviously each carburettor needs to have the same jetting, so this is the first thing that needs to be right. Also before synchronisation the valve clearances need to be right, otherwise some cylinders will be sucking for longer or shorter times than the others, even if only by a very short period of time.
I have frequently used a mercury manometer in the past, but if you watch an earlier video you will see some of the mercury has been lost. So now I have some vacuum gauges which do the same job. Basically they measure the vacuum, how hard the cylinder is sucking, measured just before the fuel and air goes into the inlet.
It is either a screw, or in this case a nut that is used to adjust the height of the slides/butterfiles. You adjust each until they are all showing an equal vacuum. In reality you can only do this at idle, but if you have skill, you can also get a view on how the vacuum is at other parts of the rev range.
As I haven’t fitted the air filters this setting will still not be completely right, but I did manage to make them all equal, and it was noticeable that the engine seemed to rev and idle a lot better.
While working on the carbs I noted that the main jets are 110s. With the cone filters I am going to run, and the very free flowing exhaust I do not think these, standard, jets will flow enough fuel at higher revs, but it shouldn't make too much difference at low revs. I am going to get some larger jets and fit them before I ride the bike, so I will need to sync the carbs again once I have made that change.
It is such fun playing with engines 🙂
For more helpful how-to guides and restoration project logs, visit our blog: www.spannerrash.com/
As an Amazon Associate, Ebay Partner and Awin Affiliate, we earn a small commission from qualifying purchases. Some of our links are affiliate links, and if you decide to purchase things through them, we earn a small commission. It costs you nothing but helps us to keep the content coming. Thanks for your support! - Auta a dopravní prostředky
![How To Fix Honda CB750 Oil Line Leak & Make New Ones [Cafe Racer Engine Leaking Oil Pipe Fix] 29](http://i.ytimg.com/vi/SbrJWLWPsGY/mqdefault.jpg)
![How To Fix Honda CB750 Oil Line Leak & Make New Ones [Cafe Racer Engine Leaking Oil Pipe Fix] 29](/img/tr.png)
Thanks for turning your frustration with this bike into our education! Nice work!
Thanks, I'm glad you find it useful. I can't wait to ride this bike :)
Glad that did the trick.
I am a lot happier now, but I think I might have a bit more to do with the carbs. I can't imagine the main jets are big enough for the, big bore, pods and free flow exhaust.
Sure sounds better! Can't wait to see her street ready.
I still run my '73 with drilled out jets ,aluminum velocity stacks and drag pipes just like my brother-in-law set it up when he bought it new.
Still scary fast!
Cheers. I think the rain and cold has slowed the pace down :( Interesting that you drilled the jets, I'm thinking of upping the size of mine in anticipation of Spring!
Looks like you did a good job bench synchronising those carbs, they were pretty close straight of the bat..
Thanks. They weren't as far out as I expected, I thought the gauges might have moved more, but when adjusting I could clearly see the changes. I am just pleased that thre doesn't seem to be an issue with one of cylinders not running now.
This is why I gave up on mine. I got a 78 cb750 as part of a trade for my dirtbike. I chopped the frame, put that hoop in with the integrated taillight and I rebuilt the carburetors. It would run… but like shit. That made me get rid of it. It’s the only bike I regret selling. They are so sick when set up as a cafe racer. Got more love and attention on that then any other 1000cc streetbike I’ve owned.
That's a great tale. I don't think I will be able to get the carbs fully sorted until I can start riding the bike in the Spring. They make grand looking bikes CB750 Cafe Racers, I do hope I can get it running well.
Hi Les, my CB has the later PD carbs, it would not run properly without the original air box or pod filters fitted, and in fact even with pod filters fitted it had issues, that’s because the carbs need a smoothed out airflow, that’s why in the original air box there are rubber velocity stacks, let’s face it, Honda wouldn’t have gone to the trouble of fitting them if they weren’t needed, now as I say that was with PD carbs, not the earlier ones that you have.
The issue with most pod filters is that there’s nothing to direct/smooth out the airflow, so the air tumbles as it’s drawn into the carb upsetting the Venturi effect.
When I fitted standard pods it ran reasonably well but wouldn’t hold a steady 70mph, it would surge, so I took the rubber velocity stacks out of the original air box and grafted them into the pods, it instantly fixed the problem and now it runs perfectly, you can see the rubber velocity stacks in this video, they protrude approximately one third of their length into the pods.
So if you get poor running I’d suspect that👍
czcams.com/video/dzyp3A1TQBY/video.htmlsi=xwEIQCkfFid3jAju
PS ignore the front mudguard, it was the original guard that I lashed on temporarily with zip ties to get me to the Dyno on a very rainy day!
Hi Mark. Great info. I have watched your video, looks like it is a great bike. The idea of the velocity stacks is great. I will see if I can find some and might try it. I have a set of PD cargb on the shelf, but I always seem to prefer the earlier ones. I am thinking about jetting for the future. From what i have seen on forums I think the 42 pilots should be right, but I have seen lots of different advice on mains. I iwll have to try the velocity stack idea first but I am thinking of getting a set of different sizes so I can do some plug chopping when I can start riding the bike.
@@SPANNERRASH I put 42 pilots in mine, standards are too small once you have taken the standard air box/filter off 👍
Cheers
Loving your work spanner rash,. i reckon a good grade fuel, optimum/v power or a good fuel additive , air filters and end can on and it should be spot on,. "hopefully",.. cheers,.
Thanks. I am sure it would be fine to ride, and good fuel is always a good option. I am thinking of increasing the size of the main jets before I get around to riding it. I suspect it will need a little more fuel when the throttle is towards fully open. If wasn't Winter I would ride it now, but while I have some time, I might as well get on with it.
Great video. Those look like CV carbs, which run notoriously poorly without the airbox because they were designed specifically to be used with the airbox - the combination of the velocity stacks and the airbox itself means that the carbs can suck in air that is all at an equal pressure & speed and is all entering the carb in the same way due to the venturi effect generated by the velocity stacks. If you run stacks without the airbox, you're still ruining the venturi effect due to crosswind and pockets of higher/ lower pressure air. No real amount of rejetting or synching will get the bike running as well as stock.
If you have a set of PD carbs, I would use those. I was able to successfully jet & tune my PD carbs for K&N pod filters + open 4:1 exhaust without too much trouble. The PD carbs work differently than the CV carbs and are much more forgiving when running stacks/ pods. The stock jetting is usually #35/#105 (pilot/main), I went up to #40/#120 and it runs like a sewing machine and moves like a scalded dog. I kept the needle in the stock (middle) position and my plugs are all looking healthy.
Thanks for the advice. I know that stock air filters are probably best, but don't look as good. Great information about using the PD carbs. I have a set on the shelf. I might try them if I have lots of issues once I can actually start riding the bike.
Very nice. I have trouble getting the brass rods in place with the choke mechanism on mine. I can do it, but I've broken a couple of those tubes. Even with different sized tubes, it can be tricky. I'm curious if you have a ballast resistor in place with the 3 ohm Dyna coils. The stock coils on the CB750 were 5 ohm, which Dynatek makes...they're the black coils. (I put them on my bike when I added the Dyan electronic ignition.) I remember watching Hackaweek's CB750 series and he used the green Dyna coils but recommended a ballast resistor go with it. I'm not even sure what a ballast resistor is.
I did find using the extra tube from my old ballance kit, made it much easier to use the synchroniser. I was surprised the brass tubes didn't just work. I haven't come across any ballast resistors for the coils, I will have watch Dino's Hackaweek video to see if it is something I should have.
Really enjoyed that. Do you have a link for the gauges you bought?
Thanks for the feedback. Here is a link to this type of synchroniser, ebay.us/g2cm5F , it is in the description as well.
@@SPANNERRASHMany thanks (apologies - I missed the reference in the description).
No problem, glad to help.