I just finished overhauling an old 1988 TrailBoss 250 2-stroke and am now having problems with it backfiring out of the carb.

I had a serious compression issue when I bought this thing (See Here) so I pulled the head and cylinder, honed it out, put in new rings and gaskets and thought I would be set........I was wrong.

After putting it back together it started and ran (idled) like new for about 20 minutes then all of a sudden it starts backfiring out the carb, bad enough to stall the engine! Now it barely starts, won't idle for crap, gets hot as hell, pings, knocks, backfires like crazy, and thats only when I can get it started at all!!!

Carb has all new parts and was throughly cleaned several times, and the carb boot is new. I'm at a loss...PLEASE HELP... before i take this thing out back and set fire to it.....lol

Thanks,
Mike

 

Could the timing have changed ? It may be too advanced.

 

Almost sounds like it sheared the key and the timing is very retarded?

 

i agree it sounds like it running lean so maybe the timing got screwed up. check the plug and see if its white (lean).

 

Is the piston in correctly, there is a front and back. Also verify the exhaust is free flowing.

 

sounds like a big air leak, cylinder base gasket, crank seals, ect., if you can keep it running spray starter fluid around those areas , when engine sounds change you have found your leak(s)...