It is hard to say what the problem is and being a used motor makes it even harder. You most likely will need a new piston and sleeve and may want to change the conrod, since it was used. What fuel do you run (% and brand)? Make sure for the next one that the motor is shimmed correctly. If the motor calls for .5 go .55 if .6 go .65 and so on. This will help you bring in the motors tune slower and ensure that you do not have this problem again. Just till you have an understanding of how the motor work. This will save you $$$$ in the end.

Regards,

DJ Apolaro