1)remove air intake from carb.. while someone cranks the engine, hold your hand over the carb and feel for suction. if no suction, check muffler, etc for mouse nest.
2)give the carb a sniff of ether to see if it will hit or not.
i have had one instance when engine would not hit on ether until, i believe, i drained gas tank and put in fresh gas. ??????? go figure