The engine is flooded when too much fuel is in the engine so that it hinders ignition. A flooded engine can backfire, burning the excess fuel, and then run fine.
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Engine Struggles to Start
If the engine is struggling to start, the starter motor will turn the crankshaft, but the fuel/air mixture will burn rather than explode. This will continue until the excess fuel is completely burned. Once the excess fuel is burnt, the proper proportion of fuel to air will return. The engine will then resume normal operation.
If the engine backfires, excess fuel in the system is being burned outside of the combustion chamber. The excess fuel may be pushed into the exhaust manifold. It can then ignite, causing a backfire.