It has to have 3 things to run, fuel, air and spark. Remove a spark plug, re-attach the wire and hold it (with insulated pliers) against any ground on the motor, exhaust manifold bolt or whatever and have another person crank the starter. If you get a fat blue spark, ignition is not the problem. A weak, yellow spark will indicate a problem in the ignition system. Look down the throat of the carb while someone else stabs the throttle (motor off) to see if you're getting fuel blowin' out the pump shooters.