What would putting a 250 carb on do?

Discussion specific to KDX models not sold in the USA
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Bitteeinit
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What would putting a 250 carb on do?

Post by Bitteeinit »

I ask because that's what my bike has. The stock 28mm carb and oil-injection have been removed and replaced with 250 carb (I don't even know what the stock KDX 250 carb size is). PO says it increased the power (and reduced fuel efficiency, obviously). would having a stock KDX 200 (US) 35mm carb (or a 220 33mm) be better? The bike seems properly jetted for the carb. I'm just wondering power-wise (torque/high-rev) what the 250 carb would technically do. I have nothing to compare it to, so have trouble knowing if it's good or bad. Bike runs fine, but definitely doesn't wheelie on demand like some claim. Stock pipe and aftermarket silencer.
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adam728
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Re: What would putting a 250 carb on do?

Post by adam728 »

First figure out what carb you have, brand, model, size. Don't worry about what bike it came off. Lots of people get it in their heads that if something came from a larger engine then it must be better. I remember way back when constantly hearing around school about running a 250 exhaust on 125's so they'd rip. If you know anything about 2 stoke exhaust tuning you'd know that's a sure fire way to kill power.

I'm not sure what was on a KDX250, and Google is non too clear. If it were a 38mm or even 36mm PWK then your bike would need different intake and airbox boots to fit the larger body casting.
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Julien D
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Re: What would putting a 250 carb on do?

Post by Julien D »

I think the later (water cooled) 250 uses a 38mm Keihin PWK. On a 200, that would be something similar to throwing a 750cc 4 BBL on a small block V8. IE:, too much carb for the engine. Poor throttle control, poor low end performance, etc. I'm fairly certain that the earlier aircooled 250 used a 36mm mikuni. I'm definitely not 100% sure on either of those carb sizes though.
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adam728
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Re: What would putting a 250 carb on do?

Post by adam728 »

Julien D wrote:I think the later (water cooled) 250 uses a 38mm Keihin PWK. On a 200, that would be something similar to throwing a 750cc 4 BBL on a 4.3L V6. IE:, too much carb for the engine.
Fixed it. [emoji106]
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Julien D
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Re: What would putting a 250 carb on do?

Post by Julien D »

Lol, right.
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Bitteeinit
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Re: What would putting a 250 carb on do?

Post by Bitteeinit »

Interesting. It definitely has a stock 250 Keihin carb (with different boots). No idea what year it's from though... I don't find the throttle control bad, but I have absolutely nothing to compare it to. I might one day "see the light" by trying out a well-tuned KDX... Very slow (5-10km/h) low-end throttle definitely isn't amazing... The rest feels good, but maybe I'm just used to it. Considering getting a stock carb/boot, but then again if I mod it out too much (next on the lost is a pipe), I might as well import a clean one from this site/US. But that would cost be roughly 1.5-2k... Plus the bike cost. Not exactly wise given that I could get a newish DRZ-400 for around the same amount of money.
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Jaguar
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Re: What would putting a 250 carb on do?

Post by Jaguar »

leave the carb on since it works fine.
usually adding a bigger carb on a bike that has a too-small carb on will increase top RPM power without disturbing low-end power.
the SR's come with too=small carbs.
(you never made it clear what size your engine is, BTW)
think about putting a good pipe on it, that will make a big difference.
Performance CDI -- KDX motocross mods -- the SR KDX -- expansion chamber analysis---> http://www.dragonfly75.com/moto/
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