98yz400

Member
Oct 22, 2006
7
0
my brother has the same bike i do, yz 400f, and last season he fell and sand went into his exhaust and slid all the way into the cylinder. so we replaced the rings and gaskets and cleaned everything out. then after we got it back together and started it and test rode it, we eventually came to the cunclusion he needed some new valves. now its all back together and he doesnt have the insane compression he had before, he can kick it like a 2 stroke without the compression release. it idles fine but it bogs in low RPM's under aceleration. we're baffled.
 
B

biglou

Misadjusted decomp cable? Shim not seated in bucket? Clearances out of tolerance? Honestly, I don't see it starting with that much of an exhaust blow-by, but I guess it's possible.

And moved to the Thumper forum. :)
 

Ol'89r

LIFETIME SPONSOR
Jan 27, 2000
6,961
45
Re-check your cam timing. It is very easy to get the cam timing one tooth off on the early Yamaha thumpers.
 
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