36mm keihin vs 39mm on built cr125

Andrew. james

I tried searching but didn't find much. I have both of these carbs and am trying to get others opinions on which is better for auto-x.
I'm running a built cr125 with a R4 pipe. I've noticed from my limited testing that the 36mm seems to have better response but possibly less top to it. I haven't driven them back to back anywhere with timing to see which is actually faster. So my question is which is better for the auto-x. I have several events I'd like to win coming up and no time to do real tests or get the kart on a dyno. I have them both set up and jetted right so I suppose I could swap them between runs but I'd rather not.

Larry Andrews · Posted: Sat Apr 08, 2006 8:30 am Post subject:

It probably depends on the track and the driver...so I'll guess that the smaller carb will be faster for newer drivers and tighter tracks.

Andrew. james · Posted: Sun Apr 09, 2006 6:10 pm Post subject:

John Denman · Posted: Sun Apr 09, 2006 6:50 pm Post subject:

You'll have more bottom end response and it will add about 400 usable RPM at the bottom with the 36mm. At the top it will peter out but still be usable RPM. It will also be less sensitive to changes in the air. It gives you a wider powerband just not as much peak power. That may help on a solo especially if the motor has a narrow band.

Mine has about 3000 RPM wide usable powerband with the 39mm so I don't need the 36 mm.
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Andrew. james · Posted: Mon Apr 10, 2006 6:48 am Post subject:

I signed up for a practice day before my next event so do some tests. The 36mm does seem like it has the potential to be faster than the 39mm.

Larry Andrews · Posted: Mon Apr 10, 2006 12:13 pm Post subject:

I think it's a trade of a bit less peak power for a more driveable engine. If that's the difference between being in-control and out-of-control, you'll be faster and probably have a lot more fun.

hth, la

Tom Barth · Posted: Mon Jun 05, 2006 6:12 pm Post subject:

A 36mm will perform better on all autocross tracks under 20 miles long... If you find it hard to believe look at any sprint track record and no doubt it was be set by a ICC 125 shifter (real kart engine) fitted with a 30mm Dellorto. My choice is 36mm for autocross for the cross engine.
I have run a 30mm on 125 TM motocross engine with success on a short track.

Been there done that with 38's & 39's. Have run big carbs at road race tracks, here, my personal choice is a round bore Dellorto VHSB 36RD with the 125 on all but the drag strips at Charlotte, Daytona and the other speedway road courses.
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Tom Barth
http://www.greenflagkarting.com/

Power is good...more power is better... too much power is just right!