Does it start then die or crank and no start ,does it crank period?Go back to basics gas spark and compression , gas is easy to check or compensate for (pour some right in the carb) does it run longer , YES , then look for a stuck shut float valve! Spark on crank easy to check again , either with old sp plug that you ground to a bolt on engine somewhere the wire will reach or screw driver in sp wire boot and with handle hold close to a ground (gap for arch). for checking spark while running well I guess youll have to have it running (any one have a better idea can chime in anytime)! If you are down to compression and last time you drove or shutdown was uneventful , then look for timing chain skip!
