82 250r.. loses all compression when warm?

[CodyRosa]

Hey guys.. just got my 250r bored, new piston, rings, gaskets, 170 compression on the dot.. runs absolutely amazing until about 45min-hour and it just kills out of nowhere.. The first time it happened it took 2 kicks to start it.. throttle wide open. I took it home and tested compression.. 160. Then i rode it today and it killed.. thought it locked up so i got picked up.. it cooled off a little and fired right up first kick when i pulled it out. Then later on at night i was riding and it did it again.. This time it had no compression at all. if it could have started.. i could have started by hand thats how bad it was. Had my cousin pull me home.. motor was basically cooled down now. I slowly kicked it and i could stand on the kicker before it went down. Tested compression and it was 170 on the dot..

If anyone has any input that'd be great.. i've never heard of a motor losing all compression and then getting it all back and starting like nothing happened. It sounds like its locking up when it kills :/

Thanks Cody

[manbearpig]

thats pretty wild.

provide some more info

what oil/mixture?
riding conditions: tight trails, mud, little air flow over motor? or long WOT runs?
plug reading?
head gasket/base gasket/exhaust flange gasket leaks?
motor mods?
anything look funny when you rebuilt the top end?

[CodyRosa]

I'm running about 28:1.. I've only rode it on open dirt roads. Last night was very cold to. I have never ran it WOT.. maybe half throttle which is basically all its got (needs to be geared)

Original plug was black, carbon and a little wet. Checked spark and it sparked great. I tossed in a new plug (br9 instead of 8) and thats when it basically had 0 compression.

No gaskets leak.. anywhere.. EXCEPT the exhaust flange. well more where the pipe connects it leaks.

No motor mods.. Reeds have little gap in them but she idles perfect and runs great.. lots of power for an 82.

Nope.. nothing looked funny.. went together perfect like it should.

[prometheus]

Sounds to me like you've got a gasket leak that is showing itself when the engine heats up. The metal is contracting and opening a gap between metal and gasket. Those can be hard to find without tearing the motor apart and replacing all the gaskets. Had a Chevy 350 small block do the same thing after completely redoing engine. I was using metal racing head gaskets and eventually they seated but it took awhile. Hope this helps. Good luck.

[CodyRosa]

I suppose I never re tightened the head bolts.. there is no leaks or any signs of oil so I guess i forgot about that. I'll try that and hope she works.. =/

[atc300r]

What is the piston to cylinder wall clearance. I think it calls for .003 thousands

[El Camexican]

If not a sealing issue I wonder if too small a ring gap could cause this symptom? If it is then there is likely other damage now.

[CodyRosa]

I'm not to sure on the clearance.. I checked the head nuts and everything is right.. I don't understand because I can ride for about an hour and it just kills out of nowhere. sounding like it locked up.

[El Camexican]

I’m starting to wonder if you don’t have a ring getting stuck in the groove. If you put new rings in an old piston there is a good probability, but if they were a matched set, new in the box then there would have to be either an issue at a locating pin, or you may have detonated it so hard at some point that the dome was distorted.

For what’s involved I would suggest that you pull the cylinder off and take a darn good look at the piston, rings and cylinder before you start it up again.

[CodyRosa]

Brand new wiseco piston and rings.. Yea i'm not sure what im going to do yet but i may pull it off and have a look. So weird that it has 170 compression on the dot.. thats what gets me. If I lost compression i'd assume i overheated it and burnt the rings.. warped the head or something but no its not.

[atc007]

Either way,it's coming back down.{ Apart}. Check your cylinder very closely for egg shape. Sounds to me like it is out of round. IF your rings look new,and there isn't apparent damage in the piston /ring area. Put a GOOD{new} or known good ring. In your cylinder. Push it in square[even] with the empty piston. You should not be able to see ANY light between the ring and your cylinder wall. If you do at ALL. It needs bored/straightened out. If it isn't this. I think your problem is gonna be very apparent on tear down.