now what?

[wheelred12]

I just rebuilt the top end on a 1990 lr250r .80 over. Got it out on a test ride and the motor locked up on me at full throtle, but it started right back up. I dont think I have the exhaust valve set right either and the tards at the dealer have no idea what the valve is so they are no help. Since you guys seem to be the experts on these old quads I'd like to see if you can help me.

I also read somewhere else that it may be the exhaust valve opening and hitting the piston since its such an extreme bore. Does this make sense to anyone?

[sblt500r]

the lt doesn't have the ame type of powervalve that a mx bike has. it has a rolling barrel in front of the cylinder, and its not anywhere close to the bore, so bore size doesn't matter.

was the exhaust bridge relived after the bore?

[ONLY2STROKES]

sounds like sblt is correct on the exhaust bridge relieving.have you lost compression?cause the exhaust bridge can stick the ring/rings real quick.

[wheelred12]

what is the exhaust bridge?

[ONLY2STROKES]

Originally Posted by wheelred12

what is the exhaust bridge?

the thin part of the sleeve/cylinder in the middle of the ex port..

[wheelred12]

So, I need to tear it down and shave some of the exhaust bridge off?

[ONLY2STROKES]

if evrything externally is good.theni would pull the topend to find the problem.

[flotek]

Originally Posted by wheelred12

and the tards at the dealer have no idea what the valve is so they are no help.

yeah youll find most of the dealership mechanics can adjust the valves on a new yfz450 but have no idea what to do on a older 2 stroke engine like yours. as sad as it sounds your better off with other riders on atv forums for info in alot of cases .sounds as though you had a seizure which is never good ,yo u need ot do a compression test then after that yo u should take your topend apart and see if the cylinder bore is in good condition and if the rings/piston is damaged .the exhaust bridge not being relived by a competant machinist is a big problem on the 250 engines ,especially ported ones .the lt250 has a rotary exhaust valve several inches into the tract .so thats definitly not your problem.