Carb Tuning people

[JJ]

My friend with the CB750F was wondering about some carb adjustments after dynoing with us. He is running CR Special Keihin's that are 33mm 65 pilots-130 mains-220 air- 1 turn out on the air screw- needle in the middle position (7 slots). He is also running a Vance+Hines 4 into 1 drag pipe, dyna coils, and separte K&N filters on each carb. He was lean idle off and way rich up top. Anybody that was there saw and heard it breaking up. He was told to drop 2 on the mains, go up 1 on the pilot, and then play around with the air screws. He thinks that the 130 to 128 is one jet size, I said 130 to 120 is one jet size. Are these carbs different in jet sizing compared to regular carbs for some reason? I told him to try his friends 117's on the outsides and 118's (or 120's) on the insides as he is running 130 all the way across. And to also put the 68 pilot back in. If anybody has any input please reply.

[legalspeed]

TLRman, HELP!! He's using specs and long words on us!!!


Mark will hook you up.

[Paul_E_D]

That was bad info you gave him. One jet size is exactly that, the change to the next available size. Most companies produce for example 120, 122, 125, 128, 130, so the change from 130 to 120 is four sizes down.

[JJ]

One jet size in carbs that I tune have always been 10 in drop. (Ex. 140-130)
They always make sizes in between even on the carbs I play with. The sizes in between have always been 1/4, 1/2 sizes to me and I haven't blow anything up yet. I was waiting for Mark, Rye, and maybe even Degsy to share some info.

We haven't done anything yet.......but he had 125's in there and did the same thing from what he said.

[Paul_E_D]

That's news to me, and I have to re-jet my racebike every weekend, sometimes more than once if the weather changes.

I'm sure others can help you more than I can...

[TLRMan]

My 2 cycle race carbs were in 10 increments, but as far as I know,
the jets for 4-cycle applications, are normally in sizes of 2 and half sizes.
Going to 118-120's may be too lean...should start in the 124-125 range.
If the low-end screw is only tuned 1 out, he still can richen it up...2 1/2 to 3 is the norm, may not need to change the pilot jet.
One way to find out is if you screw the air/fuel screw all the way in, and it doesn't make the cylinder drop out, the pilot is too big.

If he is using Dynojet jets, you will need to convert the jets sizes over to cc's/min sizes. I'm sure you can find a chart somewhere on the internet if this is the case.

[JJ]

Originally posted by TLRMan
My 2 cycle race carbs were in 10 increments, but as far as I know,
the jets for 4-cycle applications, are normally in sizes of 2 and half sizes.
Going to 118-120's may be too lean...should start in the 124-125 range.
If the low-end screw is only tuned 1 out, he still can richen it up...2 1/2 to 3 is the norm, may not need to change the pilot jet.
One way to find out is if you screw the air/fuel screw all the way in, and it doesn't make the cylinder drop out, the pilot is too big.

If he is using Dynojet jets, you will need to convert the jets sizes over to cc's/min sizes. I'm sure you can find a chart somewhere on the internet if this is the case.

He has a Keihin book with all the jet sizes but even they recommend 2-3 of their sizes if it's really rich. It goes like 130-128-125-122-120 ect...and that pilots are affected more by jet size drops than mains because of their size. As stated above he is running the same jets in all cylinders....do you think he should offset them? How much will the air screw effect the upper range? I know it is mostly low end but has small effects on top as well. I know the mains he has now the bike won't break 80 mph because it just floods it out as you could tell on the dyno. Also should he be ok with the current air jet that he's running if he drops the mains? Thanks for all the input guys...........

[JJ]

Originally posted by Paul_E_D
That's news to me, and I have to re-jet my racebike every weekend, sometimes more than once if the weather changes.

I'm sure others can help you more than I can...

Thanks for the input.....I just always used the middle sizes as fine tuning. Always looking to learn.......I probably don't run my stuff on the edge either like most race applications.