Small pipe to big pipe changes?

naz53

New member
Hello, I wanted to ask what changes are usually made if you have been runnng a clone with a small pipe & then go to the big / long pipe? Clones are new to me, thank you.
 
Most of the better carbs for unrestricted like 1.5 to 2 turns out on low speed air with small pipe and big pipes like 3.5 to 4 turns out. This setup is with 2 o-rings and reamed to .028 on low speed. You can swap out pipes and adjust this on back to back dyno pulls and pick up .2 of a hp. I've tested this many times.
WWW.RACINGCARB.COM
 

Yakattack

Member
Most of the better carbs for unrestricted like 1.5 to 2 turns out on low speed air with small pipe and big pipes like 3.5 to 4 turns out. This setup is with 2 o-rings and reamed to .028 on low speed. You can swap out pipes and adjust this on back to back dyno pulls and pick up .2 of a hp. I've tested this many times.
WWW.RACINGCARB.COM

And to back this up we just got done testing this...
Got your carb kit and finally got to compare the differences.
 

naz53

New member
Thank you for the replies, I am not sure what is going on with our local tracks this year but it looks like we will be switching to running clones. Again not sure where we will run so do not know if we will have to run small or big pipe. I just wanted to have an idea what I would need to do to switch a motor set up for one to the other. Thank you again.
 

W5R

New member
if your switching from stock breather and small pipe to aftermarket filter and big pipe, you will definately be changing the jets at least a few sizes bigger, that much i can guarantee. you will also want to bump the timing up if you are also changing from stock/steel to aluminum flywheel also
 

alvin l nunley

Premium User
This would be an interesting study.
I understand that most 4cycle people scoff at the idea of using an exhaust gas temperature gauge, (EGT) but it would tell you, “definitively”, what changes the two pipes would make in the fuel mixture needs of the engine.
And just so you know; a change of one jet size, say from a .039” to a .040” jet, is more than a 5% increase in area/flow. I don’t know for it for a fact, but I wonder if a change in exhaust pipe size could call for that big a change in the jetting. Just a guess, but I doubt it. Still, don’t take my word on that, try everything.
I could see a change in the RPM point where the engine reaches peak torque. This would mean changing the clutch to stall at that RPM. You “always” want the stall RPM to be at peak torque.
Maybe a little more RPM on top, but the engine might limit you there.
Comments, compliments, criticisms and questions always welcome.
 

alvin l nunley

Premium User
What is the difference between EGT for tuning, and using AFR to tune?
I’ve never used an air/fuel gage so I can’t answer that question, but I have used the EGT a lot and I know, with no doubt at all, that it works, and it’s available on most kart gauges on the market.
 

razorcalif08

New member
Have to agree Al. EGT and air density to fine tune. The more info the better. Just adding a open pipe with muffler= lower CHT & EGT temps and 300 +/- rpm.
 

jglenn

old member
AF guage gives much faster results than the EGT setup ( reacts faster) . switched to that a few years back on my dyno. EGT does work but the A/F guage can give you a look at your fuel curve for the dyno run. For me the acceleration of an engine on my inertia dyno is king to finding a better engine

EGT setup is cheaper than most any A/F guage worth using on the dyno

Just me
 

alvin l nunley

Premium User
AF guage gives much faster results than the EGT setup ( reacts faster) . switched to that a few years back on my dyno. EGT does work but the A/F guage can give you a look at your fuel curve for the dyno run. For me the acceleration of an engine on my inertia dyno is king to finding a better engine

EGT setup is cheaper than most any A/F guage worth using on the dyno

Just me
I've never used an air fuel gauge but I have a question.
If I use regular gas and then switch to a high octane gas, how does the air fuel gauge react?
Do both, if the jetting is right, show the same air fuel ratio? It is generally agreed that low octane makes more power in a low compression engine. If you use the EGT, just guessing, would low octane fuel show higher BGT readings than a high octane fuel.
Not having ever raced a four cycle, I can't say for sure, but wouldn't the low octane fuel show a higher BGT.
Seeing as how a 0.039 jet and a .040 jet are over 5% difference in area, it must be hard to tune the thing close enough to actually get good results for any test.
 

Hellion25

Member
AF guage gives much faster results than the EGT setup ( reacts faster) . switched to that a few years back on my dyno. EGT does work but the A/F guage can give you a look at your fuel curve for the dyno run. For me the acceleration of an engine on my inertia dyno is king to finding a better engine

EGT setup is cheaper than most any A/F guage worth using on the dyno

Just me

What A/F ratio do you go with on a clone 12/1 or 12.5/1?
 
12.2 is supposed to be optimum for 10% ethanol. these clones like to be on the fat side.. i found a chart somewhere for the optimum power ratio for different levels of ethanol. for me i like 11.8 to 12.0, seems to show the best. and Al is right, .001" of jet diameter is a pretty course adjustment.. i really like the A/F gauge.
 
I've never used an air fuel gauge but I have a question.
If I use regular gas and then switch to a high octane gas, how does the air fuel gauge react?
Do both, if the jetting is right, show the same air fuel ratio? It is generally agreed that low octane makes more power in a low compression engine. If you use the EGT, just guessing, would low octane fuel show higher BGT readings than a high octane fuel.
Not having ever raced a four cycle, I can't say for sure, but wouldn't the low octane fuel show a higher BGT.
Seeing as how a 0.039 jet and a .040 jet are over 5% difference in area, it must be hard to tune the thing close enough to actually get good results for any test.

Fuel octane in and of itself won't affect the AFR. You can have a 100 octane gas and a 100 octane oxygenated gas with the same octane and differing stoich AFRs or you can have 87 and 100 with the same stoich AFRs and different octane ratings.

It gets even more fun when octane, jetting, and AFR affect EGT and ignition timing affects AFR readings.
 

rkcarguy

New member
The difference between my .820" inside diameter header and 1.0" header was TWO jet sizes richer for the larger one on my BP motor. .038" to .040", very surprising. Both headers were long radius custom made and port matched to the head....no idea why such a difference.
Al is right, with fixed jets it's a pain. One size too lean the next too rich. I bought an adjustable main jet from stout racing, I can actually reach over under my carb and tweak it's T-handle on the fly if I need to.
 

alvin l nunley

Premium User
The difference between my .820" inside diameter header and 1.0" header was TWO jet sizes richer for the larger one on my BP motor. .038" to .040", very surprising. Both headers were long radius custom made and port matched to the head....no idea why such a difference.
Al is right, with fixed jets it's a pain. One size too lean the next too rich. I bought an adjustable main jet from stout racing, I can actually reach over under my carb and tweak it's T-handle on the fly if I need to.
That is surprising. The area difference between those two jets is 10.9%. At
 
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