Race fuel and jetting..

[wes350x]

Is it true about having to run smaller jets with race gas because of something like it burns cleaner or quicker?

Just wondering, my main jet is a 165, really dont know whats inside my motor, bought it like it is, but I do have to run race fuel. Seems small size to me, but I will do a plug chop whenever it warms up a lil more here in Louisiana

[LONG-ROD]

Usually with race fuel you run bigger jetts, cause you have more compression. Make sure you plug chop on the needle as well. Someone correct me if i am wrong.

[rustyATV]

The air/fuel ratio has nothing to do with octane rating. Unless you're running an oxygenated fuel, your jetting should be the same.

Higher octane race fuel is more tolerance of heat and resists ignition more than lower octane pump gas.

I think where this comes from is people running too low octane fuel in their engines, and the way they stopped detonation was to richen the mixture. Once they started running the correct fuel, they found they could lean the mixture back out.

Tune to a light tan plug. If you engine detonates, either use a higher octane fuel, or lower your compression.

[LONG-ROD]

Hum I had read a post that said this guy wanted to build a 250 cc engin and run motocross with it and compete aginst 400cc 4 strokes. Neil chimed in and explained how to do his build. Make a very high compression motor. He said you must give it alot of fuel because the engin will need it at the compression levels it would be at. I assumed it would suck more air, thus needing biger jet for more fuel to acomidate this. So the question is does a high compression motor suck more air?

[rustyATV]

Originally posted by LONG-ROD
So the question is does a high compression motor suck more air?

No, the compression does not affect air flow, however air flow does affect compression.

Maybe he was talking about a motor ported for high-end power, which would generate higher pressures in the upper rpm ranges.

Can you find that post?

[wes350x]

Originally posted by LONG-ROD
Usually with race fuel you run bigger jetts, cause you have more compression. Make sure you plug chop on the needle as well. Someone correct me if i am wrong.

Thanks, I really think I can go up with the main jet a bit more and lean a bit with the needle. Im having a hella time since I got this bike 7 months ago correcting the jetting. It seems to load up if I dont stay in the gas, not muck, but enough to tell. The motor is a supposed bulit by sparks 265cc?, yz piston, I almost cant wait til it breaks for a rebuild to see whats in it.

[LONG-ROD]

Here this is what Neil had to say.

Only way you'll get a 250cc engine to pull like your wanting, is with real high compression.
You will need a complete rebuild to get the bottom end tight. New OEM crank & 8 ball main bearings & seals.
With a high compression build, you dont want to take a chance on a used crank failing.
If you dont have a cool head, go ahead & get one of those so you can get the proper dome size. A dome somewhere around 18 to 16cc's should do the trick. You'll probably as well need to have larger head studs installed & the cylinder tapped to the right size. A step stud size 8mm to 10mm should do the job with the 10mm end tapped into the cylinder casting.
RUN the head stay as well to keep from blowing the cylinder off the cases.

Heres the tricky part, RACE GAS, you'll need to be sure without a dought its at least 116 octain or else you'll melt the engine down.
Dont take, my buddys down the street word for it. You read it on the can or bottle that the gas is 116 for sure.

Carb jets, big jets because with the compression you can burn some fuel.
This wont be a high rpm engine, keep them low & learn how to shift gears at the right point.
Get a CT midrange pipe, it is the best bottom end pipe you can buy other than a custom made one.
Neil


__________________
C-Leigh Racing Team
In memory of Caraleigh Pritchard
1979 1992

Team riders, 08 Extreme Dirt Track
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# 33 330R Andrea Creech, Open class 4th 08
# 81 200 Steven Fonte, 13/15 Youth Production
# 77 200 Nathen Linderman, 13/15 Youth Production

[rustyATV]

That sounds more like a special setup rather than any kind of need or benefit of the fuel.

Note, though, he says to jet richer because you can, not because you have to or should.



BTW, here's Duncan Racing's jetting guide, for reference

http://duncanracing.com/techfaq/Tech_keihin-carburetion-jetting.phtml

[All250R]

I'd have to dig deeper to cite something, but I read a while back from somewhere that some racing fuels require leaner jetting to get the most from the fuel. This is not because of the octane - but from other properties in racing fuel that differ from pump gas fuel (density?).

Racing fuel is considered superior to pump gas for more reasons than octane so the likely many differences in the specific fuel you're running need to be looked at.

[All250R]

Originally posted by wes350x
Is it true about having to run smaller jets with race gas because of something like it burns cleaner or quicker?

Just wondering, my main jet is a 165, really dont know whats inside my motor, bought it like it is, but I do have to run race fuel. Seems small size to me, but I will do a plug chop whenever it warms up a lil more here in Louisiana

A 165 in the 34mm PJ is not a lean main jet. Fuel metering is specific to the amount of air being drawn through the engine, so knowing the carb size at the very least is required to know if a 165 may or may not be in the ballpark. Now how you hit a homerun in your jetting game is not something someone from the sidelines is going to be able to tell you. You're going to have to do your own testing on your setup with your own fuel.

[wes350x]

Its a 38mm A/S carb. I may not be as lean as I thought looking at others jetting. Thought I was lean cause a 175 came with the bike, but the bike was running like crap at that time also.