you need four things for a motor to start - gas, air, spark, and timing. im sure the timing in the motor is fine, so that leaves air, gas and spark. also, air is almost always getting to the engine so ignore checking that. spark - remove the spark plug and place it back in the spark plug wire. ground the threads of the plug on the block and crank the motor. if you see a spark (its usually very faint) or you get shocked, the spark is fine. as for fuel, i cant think of an easy way to check other than removing the spark plug and turning on the gas valve and cranking the motor. fuel should spray out of the spark plug hole a little when you crank it over.