Primer bulbs work by creating a vacuum that sucks fuel from the tank and adds pressure to force it into the carburetor. Many small engines use a primer bulb to make starting easier. Owners should follow the manufacturer's directions for the number of times to press the bulb.
On lawnmowers, the manufacturer often locates the primer bulb between the fuel tank and air filter. The user presses this small, circular device to provide fuel for starting. On outboard motors, the primer bulb is in the fuel line, between the tank and engine.
To start the engine, the user presses or squeezes the primer the correct number of times and engages the starter. On many small engines, this involves pulling the starter rope, although some small engines have an electric starter.
Exposure to the elements and the materials in gasoline may cause the rubber in the primer bulb to wear and crack. If the bulb develops leaks, air begins to replace the fuel in the line, causing the engine to run rough or stall. Fuels with alcohol blends tend to accelerate this type of deterioration. The use of fresh fuel in the system and the addition of a fuel stabilizer help to reduce this type of deterioration.