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yz125 fully reved out up and down rpm

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just noticed at the track today i was sitting in neutral waiting reving my bike and fully guned it and it hits top rpm then drops (over rev) than the rpm goes up and down, like a rev limiter or something kicking in, is it jetting and could it be caused by the air screw being to lean, (to far out), totally stumped

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i think it could be the air screw i messed with that, could that cause it

You should learn what things do before you go screwing with it......

No, not the cause at all the air screw is only pertinent to your idle. Your problem lies a little lower. Most likely the float bowl inlet is slightly restricted, or your float bowl height is too low. Not enough fuel is held in the bowl for the main jet to feed consistent volume at max revs, it then starts leaning out and over revving, engine drops off and then as more more fuel is delivered...up she goes again.

Check fuel flow from the petcock, check fuel volume in the bowl and check flow thru' the float needle and seat. Check your needle is rising smoothly with the throttle. What position is the needle set at? Replace fuel filter if necessary. Is your base gasket in good condition. Might be an idea to check reedblock gasket and the inlet manifold is sealing properly also.

The chart below shows what part of the carby is utilised at any given throttle opening.

Cheers,

Chrisko.

jet-chart.jpg

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well i messed with the air screw again and did not change it, so thats ruled out, the motor just got new crank seals, the cylinder redone, all new gaskets, and piston and ring, it only has 4 gallons ran through it, and when it revs out it just goes up and down, i think its getting restricted like you said, the needle is on the 3rd clip and the reeds are v force 3 reeds with new replacement reeds and gasket, i will check the float height, but will adjusting the float height change jetting at all, and i also pulled out the plug and looked at the insulator, half of it was dark brown and the other half was white like new? never seen anything like it

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