The priming bulb on all Stihl trimmer models works in the same way, even though the carburetors differ from model to model. The purpose of the bulb is to transfer fuel from the fuel tank to the carburetor so that the engine will start when it's cold. The bulb is made of clear plastic so you can see whether it contains fuel. If it doesn't, it's usually because of a blockage or rupture.
Primer Bulb Operation
When the priming bulb of your Stihl trimmer is full of air and you push down on it, some of the air passes through a check valve and out through the carburetor. When you release the bulb, the suction it creates closes the check valve and draws fuel from the fuel tank, partially filling the bulb. The next time you depress the bulb, it sends that fuel into the carburetor and draws more from the tank. While it's normal for the priming bulb to contain some air, it should also contain fuel -- typically enough to fill it one-quarter full or more -- before the engine will start.
Primer Bulb Problems
If the priming bulb fails to draw fuel into it, the first thing to suspect is a rupture in the bulb itself. The plastic wears out and can crack after repeated uses. If you can't see any sign of a breakage in the bulb, the problem may be in the fuel line -- there may be a loose connection or a ruptured hose. Another possibility is that the check valve has stopped functioning correctly; this can happen when the carburetor is old or is soiled by old fuel. Fuel deposits and particulate matter can also clog the fuel lines leading from the tank.
Removing the Priming Bulb
Removing the priming bulb to replace it or inspect the fuel lines isn't a difficult task, but it means removing the carburetor, and that creates the potential for spillage. Disconnect the spark plug and remove all sources of flame from the area. You access the carburetor by removing the air filter housing. It's connected by screws, and after unscrewing them and pulling off the carburetor, you'll be able to unscrew and remove the priming bulb. If it's ruptured, simply replace the bulb and reassemble the trimmer. Other problems may involve some work with the carburetor.
Carburetor Issues and Solutions
If the carburetor check valve is stuck, you may be able to restore the operation by cleaning the carburetor. To do this, you need to purchase a kit for rebuilding that model of carburetor, because you need to replace the gaskets and seals as part of the cleaning procedure. The carburetor is an inexpensive part, however, and it may be easier to simply replace it. If the carburetor is soiled, it could be because you left fuel in the trimmer while it was in storage. To prevent this, either drain the fuel or add stabilizer before retiring the trimmer for the winter.