I find that if any piston engine is being truculant the best bet is to deliberately flood it. This way you know where you stand and If you crank with the mixture at idle cut off and the throttle wide open the engine will ALWAYS start. This works for every engine I have ever met ( C65 to P&W R2800 ). If the OAT is around 0 deg C or colder, however you have to be carefull about induction or stack fires. If it is really cold it is much better for the engine to preheat before starting. One trick I have used on flat carburated engines when it is cold out is to set up for a normal cold start but leave the primer out while starting. As the engine fires slowly push the primer in. The extra gush of fuel will allow the engine to pick up enough to stay running. Another trick is if an otherwise normal small piston engine suddenly will not start, it might be the MAG impulse coupling is hung up. With the MAGS off rotate the prop slowly. there should be a definite click as the impulse spring is released. If you do not hear it take a piece of wood and give the left MAG a good bash. This procedure once saved me from having to spend a night at a deserted wilderness airstrip.