I have rebuilt the carb, welbro 603. replaced all the seals, gaskets, bearings, piston ring, head gasket. Will not start. sprayed carb cleaner down the throat of the carb nothing. Put gas down spark plug hole nothing. Is it possible that a spark plug can work when grounded against the head(lots of blue spark)but not when it is installed? I don't like this engine! please help, Thanks Mike