The carburettor is where the fuel and air are mixed and fed into the engine. It is vital that the carburettor is set-up correctly and working efficiently to ensure that a good balance between performance and economy is maintained. If the fuel mixture is to weak the engine will struggle and may stall when under load, to strong and the fuel will be wasted and the oil may become contaminated with unburnt fuel causing a thinning of the oil resulting in increased mechanical ware.
The fuel and air are sucked into the engine by a vacuum caused in by the in stroke of the piston, fuel and air are sucked from the carburetor rather than pushed or injected in as they would be on a modern engine.