This component, typically a small, transparent, and pliable dome made of rubber or plastic, is responsible for manually drawing fuel from the fuel tank into the carburetor. This action primes the engine, ensuring that an adequate amount of fuel is present in the carburetor for initial combustion. Its purpose is to facilitate starting, particularly when the engine is cold or has been idle for an extended period. Pressing it several times before starting forces fuel into the carburetor, replacing any air that may have entered the fuel lines.
The correct functioning of this element is vital for ensuring the reliable operation of the associated gardening tool. Without proper priming, starting can be difficult, requiring excessive pulling of the starter cord, which can lead to operator fatigue and potential damage to the starter mechanism. Its presence simplifies the starting process and minimizes strain on the engine. Originally, engines relied solely on the choke mechanism, but the addition of this priming feature offers a more direct and efficient method of fuel delivery.