Carb size question

[Mike Rodriguez]

Will a 38mm carb be too big for solo?

I'm thinking it would be a bit too big.
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cheers!

-m.

[Larry MacLeod]

On a moto engines like Honda CR125s, 38mm is practically the standard in karting. It used to be the max until about a year ago. Although some guys (like Sheidler) still run a 36, which might be the stock size carb on a bike.

I'd have to check the Solo rules. Either they totally removed the 38mm max, or they raised it to 39 or 41.

For ICC engines, I believe they have to run in their SKUSA, or WKA max. What is that, 34? 32?


Lefty
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F125 #196
Old Birel/ hodgepodge Honda CR125
Lefty Funk
Detroit Region

[Mike Kline]

Looks like they removed the max size all together. The 2004 book says 38mm max. The 2005 book doesn't have any size limit.

-Mike

[Brian Garfield]

But, like what has already happened once or twice in the 2005 rulebook, it may have been an accidental omission. Ask Alan or someone on the KAC.

Brian

[Steve Elzinga]

The maximum was removed last year.

[Dan Davis]

ICC engines are allowed to be run in the SCCA F125 class as long as they operate within the CIK homologation specifications which specifies a 30mm Delorto carb. Even with that small sized carb, they will still pull all the way into the 14000 rpm range and the smaller carb size makes them very drivable. I think more should try a smaller carb on their Honda's to help make them more drivable on smaller auto-x circuits.

[Tom Harrington]

[Brian Garfield]

Thanks for the clarification Tom.

Brian

[Larry MacLeod]

[Dan Davis]

The last auto-x course that I have driven was low grip & dusty with fairly tight turns that induced wheelspin fairly easily when the powerband would arrive in force. I know that courses and the grip on various courses vary quite a bit from one to the other but generally speaking, engines that are not quite as peaky or making quite as high of horsepower numbers at max RPM are easier to drive by providing nicer transitions between throttle positions and a slightly wider power band much like a stock CR. I feel that the average driver can benefit from a smoother driving kart with smoother acceleration over a larger more usable power band on slicker surfaces. One way that I would try to achieve that type of feel is with a smaller carburetor and lighter reeds. I believe that the midrange feel would improve the most allowing the driver to be able to be less agressive and drive more smoothly. The driver may also feel like his input at the throttle more closely matches the power ouput of the engine if that makes any sense. For me, this feeling always gives me a better sense of control. I have only driven a CR with a 39 mm carb in the last year or so as I primarily road race but If I still have this kart/engine package by the time spring rolls around, I will show up at the first auto-x in KC with a PJ 36mm. I would expect to be able to provide better feedback at that point.

Much of my opinion is based on my experience with ICC engines in the recent past. Internally, they are very similar to a modified CR but have a much different feel when compared to the Honda's. If someone is content with the way that their kart/engine is perfoming, it may not be the best way to go but if one has a smaller stock carb lying around, it would be worth a test or two at the local sprint track to see if there is any improved drivability for that particular driver/kart/engine package.

Typically, more experienced drivers can modulate the throttle better and tune for that driving style so a larger carb could be better put to use by the more experienced driver. However, just because an engine will rev out in 6th gear, doesn't mean that it can't be made more drivable.

Another thought, if a 30mm carb provides enough flow for an Italian 125cc engine to rev to 14-15000, I wouldn't asume that the 38-39 mm carb is the reason that a CR can rev out to 12500-13000.
I don't expect there would be a monumental difference for most but races are often won by less than half of a second so small things like improved drivablity on a tight patience type of course (more time below full throttle) can make a larger difference in the outcome of a close event than peak horsepower at higher rpm ranges......something that a larger carb can help to achieve.

[Larry Andrews]

Galbraith's 'Conventional Wisdom' strikes again...

There's a lot of history of successful drivers using 36mm carbs on a CR125 - it's just not widely advertised. Probably the fastest engine I've ever driven on very tight tracks uses a 36PJ and doesn't even rev past 11500. It's an old Brad Boyer special and has enough torque to outrun any other kart I've ever been up against out of the corners. And yeah, the jug and cases have been pretty heavily massaged.

Then again, it's often a lot easier to sell some guys something "bigger".

[Dan Davis]

Thank you Larry.