The engine is said to be flooded with fuel when a quantity of liquid fuel collects in the intake manifold, and perhaps also in the cylinders. This condition gives a mixture that is much too rich to ignite.
If the carburetor has a provision for opening the choke valve when the throttle is fully open, crank the engine with the throttle open until engine starts. It will start as soon as the extra fuel is pumped out.
If the choke valve is not designed to open when the throttle is fully opened, tie or block the choke valve open and crank the engine until it starts.
Flooding may also occur on the road. If the carburetor supplies too rich a mixture at full throttle, the intake manifold may be flooded with liquid fuel, with the result that when the engine is stopped, heat evaporates the fuel and thus provides an over-rich incombustible mixture. The engine won't start until the rich mixture is pumped out by cranking.
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