Yep, Ceecrb1 is correct, I meant over tightening. I've done it before myself, and it sucks. Check and make sure nothing is warped place it back on and make it snug but not prop bolt tight. Check your main fuel line the one from the tank to the carb, it shouldn't have any leaks.
Last make sure that the carb choke is closing all the way, once all is done with a few flips you should have gas getting to the carb.
I don't think the actual engine has leaks, or you will see this (streaks of oil or residue coming from the leak source). Your problem is most likely a fuel/air feed problem.