Topic: My Carb Tuning Journey

[Bigmant]

Decided to make a new thread to document my carb tuning journey since I see a lot of unfinished threads on this subject!

So my setup is currently as follows:

Carpy’s Yoshimura style 4 into 1
Air pods from 4into1.com
120 Main Jet
40 Idle
Needle jet 5 down from top (so full rich)
Air mixture at 1 turn

Pictures of the exhaust and pods are attached as well.

I am currently dealing with a flat spot around the 3000rpm range that I have been struggling my way through. It began with the setup above, but I had the needle set at 4 down from the top. It would lag at 3k, but I could push through and it would open up at the higher RPMs.

My first change was to change the needle jet to two from the top, which would lean the mixture out. I did that, re-synced, and went for a test drive, and it was WAY worse. I could no longer push through 3k and the bike sounded like it was sucking air.

So back to taking the carbs off I went, and I went back the other way with the needle to where it is now. I put it on 5th from the top, re-synced and went for a ride. It’s not perfect, but it was better than where I started and it was easier to push through 3k.

My next step is to go up to a 125 main jet and start with the needle in the middle spot (3 from the top) and see how the bike does.

More to follow!

[Jerry Rxman Griffin aka MuthaF'er]

This will be fun to watch.

Stock carbs?  What engine/setup?

IMO, to make more than 1 change at a time will make it difficult to truly evaluate which made what difference especially without a dyno printout.

[Bigmant]

This will be fun to watch.

Stock carbs?  What engine/setup?

IMO, to make more than 1 change at a time will make it difficult to truly evaluate which made what difference especially without a dyno printout.

You’re right, I forgot to add that! Stock engine, but rebuilt all the way down to splitting the case. Stock carbs.

[PeWe]

Velocity stacks change the setup too.
My 836 with stock carbs and pods, 4-1 ...
Air screws 1/2 turn open which might help your 3000 rpm issue.

[Bigmant]

Velocity stacks change the setup too.
My 836 with stock carbs and pods, 4-1 ...
Air screws 1/2 turn open which might help your 3000 rpm issue.

Pewe, 1/2 turn in would make the mixture richer correct?

[PeWe]

Velocity stacks change the setup too.
My 836 with stock carbs and pods, 4-1 ...
Air screws 1/2 turn open which might help your 3000 rpm issue.

Pewe, 1/2 turn in would make the mixture richer correct?

Try and see.   

[Bigmant]

Velocity stacks change the setup too.
My 836 with stock carbs and pods, 4-1 ...
Air screws 1/2 turn open which might help your 3000 rpm issue.

Pewe, 1/2 turn in would make the mixture richer correct?

Try and see.   

1/2 turn definitely helped, but there is still a lag. It pulls up to about 3750-4000 now and lags before taking off. Not a smooth pull through the power range. I’m almost positive I need to up the fuel now, so next step is 125 on the main jet.

Anyone have any opinions on where I should start the needle jet? Just leave it all the way down (5 from the top) or start back at stock right in the middle (3 from the top)?

[Jerry Rxman Griffin aka MuthaF'er]

I say drop the needle. Bottom slot seems extreme. I'm running a high tech 915 CR31's at 105 (stock carb jet size) and needles in middle but I'm at elevation.

[Bigmant]

I say drop the needle. Bottom slot seems extreme. I'm running a high tech 915 CR31's at 105 (stock carb jet size) and needles in middle but I'm at elevation.

Jerry, you think start back in the middle at the stock position?

[PeWe]

My 836 using stock carbs, open 4-1 had needle setting in 4th notch from top.

Even my K2 today using same carbs, rather open Yamiya no number 4-4, airbox with KN filter (before paper filter) use needle at 4th.
Tried middle and 105 mains.

The better flowing exhaust demanded needle 4, main 115. Tested needle 3, bike middle range woke up with 120 mains but way too rich in higher speeds so I understood that needle needed to be lifted 1 notch, mains reduced.

A friend using the more restrictive HM341 had to use needle 3, mains 105.


My 836 had huge main jets.
I guess you can use 130 or even bigger.

[Bigmant]

My 836 using stock carbs, open 4-1 had needle setting in 4th notch from top.

Even my K2 today using same carbs, rather open Yamiya no number 4-4, airbox with KN filter (before paper filter) use needle at 4th.
Tried middle and 105 mains.

The better flowing exhaust demanded needle 4, main 115. Tested needle 3, bike middle range woke up with 120 mains but way too rich in higher speeds so I understood that needle needed to be lifted 1 notch, mains reduced.

A friend using the more restrictive HM341 had to use needle 3, mains 105.


My 836 had huge main jets.
I guess you can use 130 or even bigger.

Wow 130 is bigger than I thought. I suppose since I started this journey at 4 down from the top I can go back to that once I go up to a 125. If 125 at 4 down still isn’t enough, going down one more notch isn’t going to help that much so I’ll go up to a 130.

I guess that begs to ask another question. Changing jet size would seem to have more of an effect than needle height, correct? Where does adjusting the needle height really come into play?

[PeWe]

Correct needle height important for middle range. Acceleration from around 70kmh. Stable cruising around 4000-6000rpm on 5th.

Main jets kicks in later. But it is possible to run needles too lean and main jets too rich as I did on my K2 748cc ( +0.50mm pistons).
Main jets will cover up earlier, but too rich later.

My 836 in the 80's had over 140.... Small KN pods, 4-1, ported head, cam

[Bigmant]

Sorry for the delay gang! My front went flat, and had to work through that in order to get the bike on the road for another test ride.

So now the setup is as follows:
#125 mains
Needle jet at the lowest setting (most rich)(5 from the top)
Air mixture screw half turn

The bike is MUCH better. It now pulls smoothly through 3 and 4000rpm without lagging and the bike is officially rideable around the street. However, it does feel like it could pull with more power through those RPMs. It doesn’t really accelerate through them very fast. It’s an old bike, but I know it has more power in it. I am also almost at my richest settings all around, both air mixture and needle jet, so I am probably still pretty lean.

My next two jet sizes I have are a #128 and a #140. I’m not sure there is much to gain by doing 128s, so I think I will go up to 140s, see how it runs and mess with the needle jets to lean it out if it’s too rich. However I’m open to suggestions if I should go to the #128s first!

[newday777]

Most likely it's the pods giving you the results you are trying to get through.......

[Bigmant]

Most likely it's the pods giving you the results you are trying to get through.......

Yep, I am well aware! Thank you. I’ve read enough of this forum to know that’s the case. I however like them for how they look, and the bike never had a stock filter to begin with at any point since my Dad or myself have had it.

[newday777]

Most likely it's the pods giving you the results you are trying to get through.......

Yep, I am well aware! Thank you. I’ve read enough of this forum to know that’s the case. I however like them for how they look, and the bike never had a stock filter to begin with at any point since my Dad or myself have had it.

Try adding air cones inside them to mimic the stock air intake.

[PeWe]

I thought you used stacks.

If you have pods KN style on stock carbs, no problem.
That was my setup from the mid 80's.
I described that earlier. Too many think that stock airbox is a must on CB750 with the K1-K6 carbs.
I got mine fixed asap after a jetting tip from a biker friend just when I gave up  Smoothbore 29 with cracking carb boots.

If you run pods and good flowing exhaust:
Needles 4:5 from top
Stock pilot (40)
Main jets 125 and up.
My carbs had more.
You have already verified 125 way too small, so go 135.

I'm just back from a carb adjusting 270km ride. Finally almost perfect!
Pods... But not stock carbs... Real #$%*es to jet

[scottly]

My next two jet sizes I have are a #128 and a #140. I’m not sure there is much to gain by doing 128s, so I think I will go up to 140s, see how it runs and mess with the needle jets to lean it out if it’s too rich. However I’m open to suggestions if I should go to the #128s first!

Try the 128s; they should flow about 5% more fuel than the 125s, while 140s would flow 25% more! Also, don't confuse RPM with the amount of throttle opening: the main jet's effect is mostly at wide open throttle, while the needle's effect is between 1/4 and 3/4 throttle. You might also want to support the rear of the carbs like the air-box did, as the weight hanging from the rubber boots can cause vacuum leaks.

[david 750f]

Those pods from 4into1 can block the intake air holes (not sure what they are called). Make sure they are not blocked.

[gifv]/gifv]

[scottly]

Those holes are air bleeds.

[Bigmant]

Ok team, I think I have finally done it!!!

The new setup is as follows:
#140 mains
Needle jet at the lowest setting (most rich)(5 from the top)
Air mixture screw half turn
4-1 Carpy’s Yoshimura style exhaust
4into1.com pod filters

The bike runs great! Smooth pulls all the way through the RPM range, and very powerful around the 4-5000 rpm range. I am SURE there are tweaks that can be done, but I am more than happy with it. It’s a true joy to ride now and it is fun to really get on the throttle, and hear that classic sound.

Hopefully this helps someone with a similar setup. Like most have said, tuning with free flowing exhaust and pods is hard, but not impossible. Hopefully this will give someone a guide of at least where to start.

[PeWe]

My bike used Mikuni 147.5 mains!
In the 80's with its fuel.

[scottly]

Did you ever check the fuel level in the float bowls with the clear tube method?

[PeWe]

No fuel level check back then.
It ran OK and plugs looked good too. Increased mains by road tests.
But it must have an explanation. 130 something should be what expected.