Small Engine Fuel Primer Troubleshoot

Fuel primers are found on small engines like chainsaws, lawnmowers, hedge trimmers brushcutters and other small engines. Often a problem can arise where no fuel is being supplied to the carburetor by the primer bulb. If the power tool has been stored for a while you may find over winter mice have eaten a hole in the bulb or it just does not seem to work anymore. Here is an explanation of fuel primers and their function.

Primer bulbs are often located on or near an engine’s carburetor. Especially in the case of primer bulbs located on the carburetor, you can save yourself time on future repairs by replacing the bulb when dismantling and cleaning the carburetor

Do not over press the fuel primer bulb as this floods the carburetor and you will have to let it dry out a little.

A fuel primer is a small pump connected into the fuel line, with a one-way valve within it which allows fuel to move in one direction only. When the primer bulb is pressed, fuel is pumped from the fuel tank towards the carburetor, which forces the older fuel in the fuel line and carburetor back to the fuel tank.


The new fuel entering the carburetor vaporizes easily and causes the engine to start promptly.
With the inline primer, any crack in the rubber bulb will allow air to enter the fuel line, this prevents the primer from pumping, or fuel from moving in the fuel line.
When changing a primer, take note of the direction installed, the primer must be placed in the correct direction, or it will not work. As the primer bulb ages, the rubber may crack and lose its elasticity requiring it to be changed.
Another type of primer, not inline, works separately by injecting air directly into the fuel bowl, pressurizing it, and forcing some fuel out through the fuel passages into the intake passage to the combustion chamber.
Note: There should not be any raw gasoline anywhere on the outside of an engine which is in good condition. Any wet raw gasoline outside the engine indicates a
fuel leak, or a problem within the fuel system and engine. If you over prime this can cause excess fuel to drip out of the carburetor.