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Old 06-18-2013, 05:38 PM
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Default High reving when turn off choke

I have an 87 yamaha yz 250 that idles completely fine, starts up second kick every time, As soon as the choke is turned off, it starts a higher repetative rev like wing wing wing wing wing wing and will die unless gas is given. I just replaced most of the seals, before hand it would do the same thing but much more aggressive and would ramp the revs up so high and wouldnt allow the bike to be shut off, Which I assumed was an air leak. Is there anything else that could be causing this issue? Maybe something with the carb?
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Old 06-18-2013, 07:17 PM
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Sounds like turning off the choke is opening an air leak in the carb.
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Old 06-18-2013, 07:47 PM
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Yes true about the choke opening an airleak....
Few things I'd check to:
Carb to boot join isn't cracked.
Boot to head is sealing.
Crank is not grooved where the seal sits allowing a bad seal.
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Old 06-18-2013, 08:23 PM
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Ok, you pilot jet could also be partly clogged and thus leaning it out when the choke is turned off.

Paw Paw
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Old 06-18-2013, 10:34 PM
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Sounds like an air leak in the carb. Break the carb down and clean it nicely. Than, inspect everything upon reassembly to look for cracks, holes, dry rot, or anything in the boot or reed cage that would be sucking in air.
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Old 06-19-2013, 08:29 AM
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Thanks everyone. If there was a substantial air leak, is it possible for it to rev high even when the choke is on? Or is it bypassed all together. Mine has never done that, just trying to get more familiar with things.
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Old 06-19-2013, 08:51 AM
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Air leaks come in all different sizes. If you get a really large one, lifting the enrichening valve (that part that gets erroneously called a choke) can make one rev higher. the leak acts like the carburetor valve (slide) and the enrichening circuit takes over part of the job that your jets are intended for.

How does yours work when it warms up? (So far Paw's suggestion pretty well mimmicks mine. )
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Old 06-19-2013, 09:44 AM
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same as when cold. starts fine, runs fine with choke on. I can start it fine with no choke and a little gas. but the high revs are always there.

---------- Post added at 06:44 AM ---------- Previous post was at 06:31 AM ----------

Are the reeds supposed to be sealing completely when the bike is not running? There is one or two that seem to have a slight spacde beween the reeds and the cage. Could this possibly affect my air situation?
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Old 06-19-2013, 10:29 AM
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your reeds wont always be closed flat if they have anytime on them at all. sounds like you have a pretty constant air leak. what does it do if your have the throttle slide bottomed out all the way, ie throttle screw backed out to it's furthest point?

btw, you can usually flip the reeds over if you want to have them tighter to the reed block. it lasts a little while.
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