Two Stroke Motocross
Two Stroke Motocross Forum => Technical => Topic started by: Rota Ash on July 23, 2010, 07:00:33 PM
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hey do any of you guys run a pwk 41mm carb on your 250, my bike is ported/reeds & pipe's, was thinkin of upgrading the carb later on down the track, would it be worth it, it seems funny that 2t 250's have the same size carb's as a 125 and 450's have 40mm + size carbs, could our tiny carb's be lettng us down?
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In the 90's I had a 250 Maico that (if my memory is correct) ran the same oval bore 40mm Bing that my 500 had.The 250 was a torquer that did'nt display any overcarbing issues.
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Rota Ash, it's not the size that counts :P no seriously you have velocity issues working with the different size carbs, bigger isn't always better
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I've seen Honda put a 27mm carb on the early ATC250R and later moved to the 34mm on the more popular years. The mx 250's typically use 38mm, but 36mm is prevalent on trail worthy 250's. The cylinder is going to demand the same volume of air regardless of the carb size. The primary difference will be, as stated before, the velocity at which it is fed.
Larger sized carbs typically only provide enough velocity during the hi rpm's, making most of the power in the upper rpm's. They usually lack somewhat in the lower rpm's due to a lack of air velocity for those rpm's. Smaller carbs usually provide better velocity at lower rpm's generally providing better power in the lower rpm's but usually sacrificing power in the upper rpm's due to a velocity restriction.
So, it's a matter of finding the proper balance for your bikes power band and your riding style. I must say, the manufacturers do a pretty good job for the bikes primary intention and would be hesitant to change carb sizes. Changes to your motor can effect the optimal carb size, but I'm no expert on that. If you go to a larger sized carb, I'd consider putting a venturi divider on the motor side to help keep velocity up for the lower rpm's which will provide a better balance to the power output.
Basically, increasing carb size does not necessarily mean more power but more of a different distribution of power which may or may not be in balance with the engines needs. The 125's run 38mm yes, but they also develop their power at hi rpm's. A 250 running the same 38mm carb will have a better balanced power output, though typically will still see most of that power in the mid to upper rpm's. Having ridden an old yz490 using the stock 38mm carb, I find it has good power in the low & mid rpm's and falls a little from the top rpm's.
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If your looking to update your carb. Give the folks at JD jetting a try.Tell them what you got and they will pick something for you and build it with thier own coustom turned needles.
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Rota Ash I would pickup a 38 and borre it. Before my 04 cr125 was a 144, it was ported and the carb borred to 40mm according to my sag gauge. I dropped the gearing 1 tooth, you have to carry momentum in corners, to keep it pulling hard, stop and pivot wouldn,t work. I,m thinking about putting that carb on my 06 cr125, for this weekend , the jetting should be the same right ?
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I have to say, Grumpy, that was a damn good post. I learned a lot on that one, and now it makes sense to me why these little 125s are running carbs that are 2mm bigger than a 501.
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I talked to a tech. at Boyesen today, he said the larger carb will run richer,because of more fuel,air,mix,velacity is greater.