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99 YZ 250 knock after rebuild

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Did the bottom end and replaced the piston in this motor. It starts and runs like a champ but when it gets hot there is a knock at idle. It disappears when I give it gas. Its a wiseco piston. The cylinder has no marking of A,b,c,or d neither did the piston that came out. I know that year came with 4 different size pistons. I'm assuming the piston I got in the kit was SIze a and that the knocking when its hot is from the piston being too small. Any feedback would be greatly appreciated.

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Could it be fuel mixture causing detonation at idle? Try to adjust your air screw (Clockwise) to increase fuel richness at idle.

Any chance that your power valves are hitting the piston? Pull off your pipe and look at the piston through the exhaust port. Are they connected and set correctly?

If these are ok, call wiseco and ask them about the piston size.

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Forged pistons actually expand with heat, so if it were piston slap, it would be more evident upon start-up or when cold I would think. Does it sound like it could be detonation? That's more probable at higher temps.

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Wouldn't the cylinder bore increase in size also when it gets hot? The jetting is stock and we ride at 0-1000 feet. The ratio is 40:1.

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It's only audible when it gets super hot and idling. I went through the clutch 3 times thinking I put a collar on in the wrong order and in the wrong way.

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i have the same prob on my 05 and i think its the topend because i put in my second piston after the barell was replated and i think its gone too big for the piston, this noise isnt costant either which bugs the hell out of me. it happens when the bike is just about to shut off and then when i rev it it stops. does this sound familiar jeremyzdead

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@ Greg Y I got a skid plate on it but its tight and lock tite-ed. @ 2strokefanatic sounds like you said. Taking it a part and measuring the bore so I can get the correct sized piston.

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skid plates make for some very odd harmonics. they all seem to smooth out as rpms increase.

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ya let me no how u get on, i am gonna do the same, prob have to get the barrel replated, not to pushed about gettin an oversize piston because it could be tight in 1 spot and lose in another spot so it would slap

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I had the same thing happen to my '00 model. Pulled it apart and took the cylinder & piston to machine shop and had them check it with mic and bore gauge. The cylinder turned out to be a D and the piston was a B. Ordered the correct piston and the issue was resolved.

I'd do the same if I were you. It won't be good long term with that piston flopping around in the bore.

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