Re-jetting a carburetor for a modified outboard.
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- čas přidán 7. 06. 2024
- This is how I re-jet the high speed jets in a two stroke outboard. Please note that factory jets on factory motors are normally spot on.
Re-jetting is needed when changing to larger carburetor or adding a carburetor from a different motor, modifying etc. - Krátké a kreslené filmy
Nice video
Helpfull! Thank you.
Hey Glenno, Have you had much to do with 25hp Tohatsu 2 strokes? Is there anything you can do to the stock carb to gain some rpms or power? Cheers
I’m looking to try this on my Tiller Honda BF 15 outboard. I’m just a hair short of being able to plane out all the way on my skiff
Are you planning on using the same carburetor? Re-jetting the original carburetor alone will not work, you need more air as well (bigger throat). The original carburetor will already have the correct high speed mixture jets for the air flow of that carburetor.
If you find another carburetor with a bigger throat that fits your bf15 be very careful not to run a full throttle for more than 1 min until you check the plug burn colour, you can blow your motor very quickly if your running lean.
For starters I would leave the carb and try (one at a time) a lower pitch prop, motor height, try a fin on the motor, weight distribution in the boat.
The last thing you want is to damage your motor.
In my video I was putting a 9.8hp carburetor on a 5hp. After lots of playing with main jet sizes I probably got about 6hp.
Hope this helps mate, it’s so annoying when your nearly on the plane but not quite.
Another option is a bigger motor as it could need more torque as well.
Cheers
I have a 1980’s 7.5 mercury and needing just a little more speed to plain out but also get somewhere what’s your advice on this I don’t have time or money to go outright buy one but if I could just get it to push like a 9.8 or 9.9 I’d be happy what’s your advice on this?
Great vid, would you happen to have any advice for increasing HP on a 15hp yammy 4 stroke single carb 2 cyl I was thinking on removing restrictor plates but not too sure , any thoughts?
Hi mate,
I am not 100% sure about the Yamaha 4 stroke, haven’t played with one yet.
If it’s the same power head as a larger hp, I would definitely start by seeing what differences there are in the two.
If it’s restrictors, timing etc I would do it.
If it’s a big deal and the cost is too much I would look into getting a bigger motor.
Cheers, sorry I could not help more.
Can you do a video on setting the timing advance on a Nissan/ tohatsu 18hp 2 stroke?
Hey mate, I appreciate every video you post . Simple question, how do I turn my merc 15 super into a 18 ? I have seen vids of making it able to open the throttle but do I need to make the jets bigger ? I appreciate you always replying mate, cheers 🍻
Hi Mate,
Thanks for your encouragement 👍
If you look on my Channel, I put up a video comparing 9.9hp - 15hp and 18hp, I believe I show the stator in that video.
So it’s the throttle cam on the stator (it tilts the carb open) and advance the timing slightly and your at 18hp. It’s hard to tell the difference though in speed between the 15 super and the 18hp.
Cheers 🍻
Aussie Boat Guy 😎
hey G, I have a 2001 mercury 15hp 2 stroke. If I did this to my jets would it increase my Hp?
whats the little rubber cap for?
Unfortunately not. You are better off seeing if you can get the 5hp carburetor if there is one for your capacity motor. Also you would have to make sure if there are any other parts you need for the extra hp.
Cheers,
Glenno
My hero I drilled mine way to big and didn't document my process so I had no idea what size to go to have you ever soldered a jet then re drill it
Hey mate,
Yeah it’s easy to go too big.
I have not soldered and drilled out, I have mates that have done that to lean their original mix out though.
Cheers 🍻
So - what is the difference between the 4hp mariner/tohatsu and the 5, parts wise? I see on eBay the carby listed as the same - but is this incorrect? Or is there a restricter one can remove? Different ports? Cheers
From memory the carbs are different, I will look into it and get back to you.
Cheers
The carburetor parts numbers are different from tohatsu, so I’m assuming the 5hp has a bigger throat. I have only had 1 4hp tohatsu and didn’t check that carb against a 5hp.
Interesting the main jets in the 4 and 5 seem to be the same size.
I would try and go to an outboard wrecker and check it out.
Howzit from south africa 🇿🇦 mate loved the video , by any chance could you please tell me where I could find information on modifying a yamaha 60hp 2 stroke , I hear there is lots of tuning potential but no body can tell me where or what I should do 🤦 please if you could direct me to a forum or something I'd love you forever !
Yep, a lot you can do to the 60hp yam 2 stroke. I believe it’s a de-tuned 70hp anyway. The 70hp itself is very modify-able.
I would look at scream and fly forum.
Check out Boatracingfactsforum and also extreme tiller steer page on Facebook.
Hope this helps mate 👍
Cheers
When the engine is closed I install the power is weak, but when the engine is closed I release the engine power is perfect
Sounds like something in your keys still.
Cheers 🍻
Aussie Boat Guy 😎
@@25hpboatracerman i understand now,thank you
cheers🍻
Hi, I just bought a 1993 20hp Yamaha autolube. Can I gain 5 horsepower by rejetting and or removing any restrictor plates?
Just wondering if it is the twin carburetor model? I know that people say they gain speed but it’s only marginal sometimes from the 25hp upgrade so be carful how much you spend doing it. From my understanding, the carburetors are different and I believe and the 25hp carbs have a larger throat. Cheers,
Glenno
would running a motorcycle carb with velocity stack be better then up jetting it?
Hi mate,
Jetting up isn’t really a thing, you need the correct jet for the throat size of the carb you are using. I was re-jetting that carb because it was off a much bigger motor with and had a 17mm throat instead of a 13mm throat.
Mates of mine have used motorcycle carbs on outboards before and they do work but no better than bigger outboard carbs.
Velocity stacks are a good addition for sure to any carb on an outboard in my experience.
If you are after more hp you need to get more air and fuel, the more air part is just as important as the more fuel part. Just adding extra fuel will loose power as it will run rich.
Hope this helps mate.
Cheers 🍻
Hi in an earlier vid regarding tohatsu 8 Vs 9.8 differences, you mentioned that there was a difference with the exhaust plates. Can you describe the difference and do you think 8 hp plate could be modified to 9.8 spec. Many thanks in advance.
Hey mate,
Can’t remember the vid, was that actually the 9.9, 15 and 18hp comparison vid?
The 8hp and 9.8hp have the same exhaust plate but the heads are different and also the carburetors are different.
The 9.8 head is a high compression head, I have shaved an 8hp head down to match and it worked the same.
The 9.8hp carburetor has a bigger throat.
To tell the truth, the difference between the 8 and the 9.8hp performance isn’t massive.
Hope this helps 👌
@@25hpboatracerman Thanks for the quick reply. I just had a look on parts catalogue and there is a different plate for the 9.8. As for the carb I thought it was the same, as all eBay listings say 1 carb for both, but catalogue does quote different items. I was going to mill heads myself, what did you aim for about 150psi?
@@Phantom-mk4kp
The stock 9.8hp normally has around 140psi. Definitely has a bigger carburetor throat, from memory it’s 17mm, I can check tomorrow, think I have one on the shelf. The 6hp and 8hp run a lower compression.
Interesting about the exhaust plates, I will look into that, wonder what the difference is?
I have fully rebuilt about 4 x 9.8hp Tohatsu, about 4 x 8hp and had many 6hp blocks apart along with heaps of part motors and never noticed a difference with exhaust plates, not saying there isn’t, just sad I didn’t notice if there is.
Might rummage through my old blocks and see what’s there.
I would be really appreciative if you can me posted on what you find.
Cheers 🍻
@@25hpboatracerman Just found your channel and seems you may have an answer for this one: I have looked into the 8 vs. 9.8 Tohatsu differences before and I remember reading on a forum that the two had different porting as well... Have you come across that? You think it's true?
@@AntiVaganza
Hey mate,
Not 100% sure that the 9.8hp has different porting, it may have.
The big differences are the carburetor and the head, I have added the 9.8 carb and head to an 8hp in the past with great success.
Cheers 🍻
Que lindo sería si fuera traducido.
On a 1999 merc 120hp 4cyl 2stroke. Uses 2 carbs and the lower carb has a bigger jet then upper carb. Why are the jets different sizes from upper carb to lower carb? Why don’t both carbs run the same size jets? Any information would be much appreciated thank you.
Hey mate,
Yeah that’s a weird one ?
One thought I have is I think that particular motor idles on two cylinders of the four (for trawling and fuel saving reasons) also an interesting feature. So the bigger jet may be something to do with that to lube things etc at lower rpm.
I would definitely be looking into if that’s how it’s supposed to be by contacting a Mercury mechanic.
Does it seem to run nicely?
Cheers
@@25hpboatracerman ya thing rips but I’m doin a refresh as it’s my off season here in the north east the carbs themselves are worn out so I really need new ones sooner then ltr so I want to go with flat slide carbs something with a little more control and crispness rather then these crude old school butterfly carbs and just want to understand why they did that so i can get a understanding on a new carb setup.its a 120hp 4cyl 2sttoke and the top carb feeds top two cyl and bottom carb feeds bottom two cyl. Definitely operates as a normal carbureted 2stroke at idle so on an so on that’s why I’m a little stumped on why the top carb uses a .088 and the bottom carb uses a .090 jet. This is in my 13ft jetboat. And the manual says that’s correct jetting so I know it’s factory.
Hey mate, I got a 2006 yamaha 15hp 2 stroke. Can the jets be enlarged on it to make more power? Or is that not a thing.
Hey mate,
Unfortunately not a thing for the Yami 15 two stroke.
I was re-jetting as that carburetor came off a 9.8hp and I was putting it on a 5hp. The jets where to big for the smaller capacity motor. The jets need to match the amount of air entering the motor to mix the correct amount of fuel to air ratio.
Cheers
Cheers.
@@25hpboatracerman Ok cheers man 👌
Hi Mate , I own a Tohatsu 9.8hp 2 stroke .. Is there anything n can do to the carb to increase HP ?
Hi mate,
Unfortunately not much can be done, the 9.8hp is really a modified 6hp and is pushing a fair amount of hp for its capacity already.
cheers 🍻
Have you had anything to do with Parsun 9.8 ?
Hey mate,
Only spent a short amount of time with a mate who had one apart on the bench.
Looked like a bit of a 9.8hp tohatsu copy but cheaper.
I honestly haven’t played with Parsun outboards too much.
Cheers 👍
Hey mate, I got a 5hp mariner spark plug is looking orange like your picture. i am wonding how to check what size the jet is already so i can know what size to change it too. Thanks
Hey mate,
Orange or brown is normally ok, they should not look fully burnt.
White from too hot or overly wet with fuel is the issue.
You need to put a new plug in and give it a good full throttle run then check plug colors, old plugs are normally burnt or colored.
The high speed jets normally have a tiny stamp on them to show what size they are. Once you know the size you can check if that’s the correct jet for your motor.
If your looking for more power, changing jets won’t increase power without increasing the size of the carburetor air hole which is a fair bit of work, you can loose power too if it is slightly wrong.
Is the motor running well?
Is it the Japan built 5hp?
Cheers 🍻
Aussie Boat G
@@25hpboatracerman Thank you so much for your quick reply, yes it is made in Japan but bought second hand so I'm not sure what the owner before me did to it. The motor runs very well and with one person in my small 2.2m fiberglass boat can get on plane and sit at 13knots and still be on plane cruising at 7knots. However when me and my mate go together on it together it tops out at 5 knots. My prop size is 7.8x8x3 and was looking at getting a lower pitch prop but unable to find a good one. Thanks
@@danielstewart8255
Nice 👍
Yeah, two people will slow it down for sure.
If it’s running nice then the mixture can’t be too far off.
Trying different props is a good idea, try some outboard wreckers to see what they have.
Cheers 🍻
can you do this on a yamaha 4 hp 2 stroke to increase the power to 5 hp?
You wil get more power not speed
I have a Suzuki DF6A with 6HP 4st. The boat with load wont plane, engine RPMs won’t increase. Already changed the propeller and I It helped but I would like to “increase engine power”. Is this possible? Unfortunately changing engine to a 9.8hp is not an option as the engine weight literally the double and I can’t move it by hand.
Hey mate.
Sounds like you do need more hp.
The 9.8hp tohatsu 2 stroke is the same weight as your 6hp four stroke. I know the 9.8hp 4 stroke is heavy but if you can go two stroke, you will have the power and weight your looking for.
Unfortunately your 6hp in not that easy to modify, I believe it shares the same block as the 5hp.
Hope this helps.
Cheers 🍻
@@25hpboatracerman thanks for answering!! Due to contamination requirements I’m not allowed to use 2T engines in the lake. Will have to continue using my old and good suzuki 6hp! Cheers!!
I have change the mainjet from 0.82 to 0.99 and i have a neadle atomizer jet with 8 holes and i want to put one with 12 holes.. hoping to bring more air to more fuel and finaly hope for more power..
Changing the main jet from .82 to .99 will make you go slower as it will flood the air fuel mix. You cannot just put a bigger main Jet in without making the throat bigger.
The main jet size is strictly related to the throat size.
Best thing to do is try the .99 and get a feel for what it does.
Cheers 🍻
@@25hpboatracerman i will put a nozle atomizor with more holes and try to enlarge the throath with 3 mm diameter
@@25hpboatracerman i also find a 9.9 yamaha carb that will fit my 5 hp and i thing if i put that carb will give me some power to go plane
may i ask?
Sure
What is your recommendation to get more power out of a 1986 Yamaha 8HP 2 stroke?
unfortunately with the 8hp Yamaha 2 stroke it is already tuned pretty well, it shares the same block as the 6hp.
I don’t have any recommendations on how to easily get more power unfortunately. You can always port the block and source a larger carburetor and tune it that way. that is a heap of work with only possible results, you can easily loose power too.
sorry there is not a bolt on option mate.
cheers 🍻
@@25hpboatracerman How about Boyesen reeds?
I have found Boyesen reeds to not really make a difference, especially on small hp motors.
they do add some small gains on a motor that is modified in my experience and when fully modifying a motor all gains add up.
cheers 🍻
@@25hpboatracerman Thanks. Reason I ask, is the small
'cocktail racers', use OMC/Evinrude/Johnson 2 stroke 6hps and they use them. Are you familiar with that mod to those motors? Or, do they do something else too?
Hi mate,
yep, familiar with it. I do get adding boyesen reeds for racing, if they add 1/2 or even 1/4 mph to your boat speed that can be the difference in winning a race.
As for actually noticing the difference in speed or extra hp I am yet to be convinced.
cheers 🍻
Aussie BG
Hi, by enlarging the hole in that tris, you will achieve exactly what
By drilling the main jet I added slightly more fuel to the air fuel mix entering the engine. Keep in mind that carburetor was of a 9.8hp tohatsu (twin cylinder) and I was putting it on a modified 5hp tohatsu (single cylinder)
I don’t play with the jetting on a standard outboard as they are already correct.
If the fuel- air mix is too lean you will blow the motor in a matter of minutes, if it’s too rich it will not have any power, it’s got to be ridiculously correct.
Glenno
@@25hpboatracerman hi i have a marine engine tohatsu 5hp 2t there is some possibility with this engine to increase the power da it can be said that the carburetor looks exactly the same
The 9.8hp carburetor looks nearly the same but the fuel pump hole is in a different position and needs to be die grinded to adjust its position. The 5hp tohatsu won’t accept the extra fuel and air mix without being ported and I also put a high compression head on as well.
Definitely not worth the effort, I think I got maybe 7hp out of it after a lot of work.
I would just buy a 9.8hp tohatsu, they are awesome
Cheers,
Glenn
What function that rubber on top pilot jet
How did the motor go after the re-jetting to .89
Wondering if I can improve the performance of a 5hp yamaha 2st
That carburetor (originally off a 9.8hp tohatsu) was going onto a tohatsu 5hp single cylinder two stroke motor. The throat size of the 9.8hp carburetor was about 3mm larger in the throat than the 5hp carb.
I definitely got more power and speed once getting the jetting correct.
Issue was that starting with the 5hp I might have only added 1 to 1.5hp so not a massive difference really.
Definitely got onto the plane easier and added about 5kph to top speed.
Was fun and games, I have kept the carburetor but sold the motor hoping to get another 5hp block and port it to allow better flow as the ports are very small in the 5hp, I feel this is why I didn’t see better gains. A high compression head would help a lot too with torque.
It’s a fair bit of work to see decent gains from a 5hp if 5hp is the biggest hp from that produced block (some 5hp are the same block as a 6 and 8hp like Johnson and old Suzuki)
With your Yamaha if it’s the two stroke single cylinder your probably in the same boat as I am in with the tohatsu.
Still fun if you like tinkering.
Hope this helps mate
@@25hpboatracerman cheers mate thanks for the reply.
@@25hpboatracerman is cool if you can put the 9.8 carb on the 5hp engine without modification!
@@_.R.F.X.L._
Yeah, it would be good,
Unfortunately there was a heap of mods.
Is your motor a Yamaha?
Hey matey im in qld just wondering if u can tweak up a 5hp carby for my daughter
Hey mate,
unfortunately not much can be done to the 5hp to gain power, the ports are so small that they don’t accept more fuel and air. I tried a bigger carburetor on one (tohatsu-Mercury) in the past, tuned all jets etc and ended up with the same output.
cheers 🍻
@@25hpboatracerman if i port the air intake 1 mm on each side or 2 mm with biger mainjet do you think will gain some power?
@@_.R.F.X.L._
Hi mate,
Tried that, no real noticeable power increase I went from 15mm throat to a 17mm throat and played with heaps of jet sizes.
I think you would need to go up to about 20mm to notice a big difference but the little 5hp ports are so small they cannot accept more air fuel mix anyway. You would have to port the block first.
Ps: I am in the midst of doing just that, should be a vid coming in about a month, lots of work, faster?? We will see.
Cheers 🍻
Any tricks on a 2006 18hp 4 stroke tohatsu?
Unfortunately I have not played with the 4 stroke 18hp.
I wonder if it’s the same block as the 20?
@@25hpboatracerman unfortunately it’s 9.9, 15, and 18hp that all shared the same block then later they went to the 9.8 which was an 8hp block and the 15hp, and the 20hp shared the same block. Mine actually started at a 9.9 and I did the cam and carb making it an 18. We’ve got a lot of 10hp lakes. Now with the new 4 strokes you just take the restrictor plate out and switch to the 20hp ECU! I’m running a 9 pitch stainless Solas now with added cup. I’m starting to think I’m maxed out lol
I have a 6hp mercury 2 stroke 2001 and i am trying to convert it to 8hp
Am guyssing i iwll need to get a carb from an 8hp and swao thr carb over
Hi mate,
I would put the 9.8hp carburetor on if your changing it anyway.
That probably give you just over 8hp.
Cheers
Aussie Boat Guy 😎
@25hpboatracerman
Yeah I suppose I could try that
See what happens
What function rubber on top pilot jet..
Late answer, but: It closes the direct entrance of fuel to the low speed jet due to that the low speed jet must take away fuel by a short cut channel to the main nozzle until the vacuum of the venturi (at higher rpm) sucks enough to let fuel come through the main jet as well. It is a rubber plug to get a simple way to access to the low speed jet for maintenance. Removing the plug will let your motor start hard (if any possible) and slows down the low and some mid range power a lot.
better use a set of nozzle reamers...
thought about this as well, 👍 but the benefit of using drills is that you get what you drill. In case of reamers, you have to check frequently, hoping not to ream a little to much. By the way; this shown carb is a Yamaha development. The thread size of the nozzle is M8 x 0.75mm If you are rejetting carbs like this fequently like i do, you can make you an adapter M8x0.75 mm OD to M4x0,7 mm bore using i.e. Zuendapp jets which you can pick from internet in larger sets for realy cheap.