Serious carb question for the racers and builders

[goodman24x]

Our engine is a ported airsal 200. We started with a pwk33 keihin. I can't afford to buy a larger and smaller carb to test.
Would I see any gains from a 35 or would it be too much for a 200 motor.
Would a 30mm work better than a 33? For tight supercross racing?
Also has anyone tried the koso 32mm carb that is a keihin copy?
Appreciate any help, looking for real facts not hype.
Thanks to anyone who has good info?

 

[Awk08]

i'm gonna say a smaller carb will retain/regain your bottom to mids that is required for your type of racing, over the larger carbs that favor mid-top end power.
i'd be tempted to try a 28mm PWK over the 30mm OKO's
or any of the other knock-offs

there's also a cfm calculator out there, when 200cc and say 8500 rpm's is punched in, it suggests a 30mm carb being able to flow the cfm's a 200cc cylinder demands at those rpm's, a raise in the rpm's showed no larger carb.
not until i punched in 240cc's did it jump to 35/36mm carbs.

you'll need to convert cc's to ci's (200cc's = 12.204748819 ci's)
i used 90% as the volumetric efficiency......

The math formula is:
CARB CFM = Cubic Inches x Max RPM's / 3456 x VE%

here's another one i did not try, let us know your results of both calculations ??.....

http://www.ehow.com/how_7822268_calculate-air-flow-stroke-engine.html

 

[Braaaptor]

I run a 34mm Mikuni on my Blaster, which has a ported stock cylinder and a modified head. The carb is really too big for my engine. It screams up top, but my low end is weak. There isn't sufficient air velocity moving through the larger 34mm carb bore at low RPM's to generate good power. The slower moving air can't create as much vacuum at the needle jet, and the engine leans out unless you compensare with a richer needle. Then you get into tuning problems. A 34mm carb works on a ported Blaster engine, but it seems to make low end power suffer.

And of course, on a tight MX track you need strong low end/midrange power. So, I would definately NOT go bigger than 33mm with your carb on an MX engine. Now if you go smaller than 33mm, I am wary of knockoff 30-32mm Keihin carburetors. Some are good. Others....not so much. Lots of them have poorer quality castings than their Japanese counterparts. If you can get a good deal on one, you can often make it work, but use genuine Keihin jets if possible so at least tuning will go smoother because you will know exactly what size orifices you are dealing with.

 

[Braaaptor]

You may want to consider trying a Keihin PWK 28mm, as awk suggested. Lots of these carbs can actually be bored out 2mm. You could get a genuine 28PWK and have it bored out 2mm oversize to 30mm. I know KOR and others bore stock Blaster carbs. If the Keihin has enough meat in it I don't see why it couldn't be bored out as well.

Some people have said Keihin PWK carbs are difficult to needle for a Blaster engine. CT racing claims to have developed a special 'titanium' needle for this carb which really helps it work on a Blaster. Of course the price is ridiculous ($ 220.00 last I looked for carb and needle) and Im leery of the claim, but you will probably have to play with swapping out readily-available needles a little until you find the right one. Maybe get a CEL and DGH and see which works better for you.

 

[DUALSTROKE]

I agree that for motorcross racing a 33 may be too big for your needs.Braap and Awk have good info on the pwk28.Boring a pwk28 to a 30mm will be the best for your riding type.imo I also would stay away from any oko,etc knock off carbs.Stick with genuine keihin and genuine keihin jets and needles.