my friend has a bike thats kind of built. ported, coolheads (18cc), tors elimination, tiger built crank and gears, so on and so forth. He has rebuilt his engine twice in one month, both times the spark plug in the left cyl was toasted and there was a hole blown in the piston. Any one have an idea what the problem could be? The first time the bike went out the crank went out and the plug was blown, and the last 2 times only the plug has blown.