blew cylinder after rebuilt

[wslute]

hello all, i just finished rebuilt]ding my 650. put about 10hrs on the rebuild and it eat the rear piston. now it didnt mess up the cylinder wall it just eat the lower ring and every thing below that on the intake side. im just trying to figure out what happened. i checked the plug and it was carbon up but still sparked so it was getting fuel and the cylinder looked lke it was getting fuel not washed out. but one thing i did notice was that around the base gasket under that cylinder jug was some dirty fuel. now im thinking the gasket blew out but would that cause it to eat a piston? it kinda make sense since the scores are only on the intake side. ive been have tuning problems since the rebuild but ive been running rich almost alittle to rich you think this would cause the gasket to blow out?
just wondering what you guys think and if you could give me some advice.
thanks

 

[6KJR]

Always do a LEAK DOWN test on a new rebuild. Always clean/replace worn carb parts, always insure OIL pump is working, ALWAYS delete grey fuel lines.

Who rebuilt the engine?

Theres a few more things you can check too but those are of great importance on any engine.