Ok my son was given a 1991 cr250 that needed repairs so I started checking it out and found very low compression so we pulled the head and the piston was melted on the exhaust side at the rings so I ordered a new top end kit from Namura Technologies after putting it together it started fine and I ran it about an hour and then put it up for the night. The next day we ran it for about 2 hours and then the flame arrister came off the silencer and all power was lost the silencer did not have any packing in it. I pulled the spark plug and it appears that the piston must have hit the plug just enough to close the gap so I verified that the plug in it was the wrong one. I then pulled it back apart and the new piston was melted on the same side (exhaust pipe side) but a different spot lower portion of piston melted off please I mixed the gas 32:1 and was running it easy for break in

 

It sounds like it is lean, it may have a vacuum leak. Or the jetting is way off. I would look at the intake boot and gaskets (reed cage area) if you dont find any leaks. start with a much richer main jet and work from there. im sure some others on this forum know more about 2 strokes(than me) and will be able to help. good luck.

 

look in your repair manual and see what it calls for for stock jet sizes. then pull teh carb and see what sizes it has in it. ans look for air leaks of corse. how does it run when it does run?

 

If you don't find an airleak in the obvious places try looking at the crank seal on the stator side.

 

What everyone before said is a good start. If it isn't jetted lean or have a air leak. You might have had to hot a plug. Also the ignition could have been advanced too much.

 

good to see everyone has not forgotton the 2 strokes.. yup, its either lean or not getting enough coolant due to low water level, broken pump or clogged passageways.
1. Do you get a nice blue puff of smoke at shifts? You should get a tiny bit of smoke.
2. Can you put your hand on the cylinder after a 10 minute ride? Should be almost to hot but not burning hot
3. what color was your plug? should be tan but never white
4. A 2 stroke running to lean will have a strange smooth idle, is it tough to start?

Hope this helps, Joe