How a Small Engine Primer Works

A small engine primer is a bulb of plastic with a hose running through it. This hose feeds from the gas tank to the engine carburetor. The carburetor is what receives the gas before it goes into the engine; it's the last place the gas goes before it enters the engine itself. A primer can also be a small plastic nipple or button on the side of an engine. When pressed it draws gas from the tank and lets it in to the carburetor. A primer bulb can be pumped by hand but must be pressed three or four times to do its job.

A primer will pump a small amount of gas into the carburetor. Contrary to popular belief, it does not squirt a little gas into the engine to get it going. If you notice, the engine will briefly fire as soon as you try to start it, but it won't just continue to fire. This is because there is no gas in the carburetor. So, when the engine sparks and ignites the gas inside the cylinder, it can't continue to run on its own. A primer sends gas into the carburetor so it can create a fuel and air mixture that is ready to go right into the cylinder and keep the engine running.

Small engines do not have fuel injection systems like larger cars. They use the force of the engine to pump gas and air into the chamber and cause combustion. When small engines sit for a long time, the gas seeps out of the carburetor and fuel system, so at startup the engine is bone dry. The carburetor needs to be filled with gas (via the primer) in order to get the engine started and to begin the process of pumping fuel into the engine.