[yz125rider21]

I just redid all the cylinders and pistons because one cylinder was having the problem below and after all that the same one did it again. I cant seem to figure out what might be causing this so I added the pictures to see what you guys think it could be.

 

[glockm22]

Is this the front cycl on the engine? Can be caused by many things, Lean condition, out of time from the crank being bent or vacuum leaks or carb fuel pump bad.
I had a a customer with a sl750 I had to install a fuel pump on because for some reason the front cycl would run lean and burn it up, once the extra pump was added he never had the problem again. Try going to PURELY TECHNICAL online with and they have many fixes and recommends for ski's just look yours up. Good luck, know the frustration.