RM 250 motor problem
 

hey, my friend has an rm250. he took it out from being completely rebuilt from a shop. was driving it and all of a sudden it quit. took out the plug and there was no gap. it was like warped to make no gap (understand?)then put a new plug and drove for about 30-45 minutes. ran great. then big backfire and the bike quit and the same thing happened to the plug. bike now has less compression but still has fair compression. the gas mix they used was very rich. and help or info please write.