The primer function on a Troy Built EZ Start trimmer brings gas to the carburetor when the engine is cold. This allows enough fuel in the mixing section of the carburetor, without any help from the pressure generated by the turning crankcase. When this bulb stops filling, something inside the fuel system is suffocating the lines.
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Dirty Fuel Tank
The most likely culprit for this problem is a dirty fuel tank. If any impurities in the fuel, weeds or other debris float around in the gas tank, when you push on the primer bulb, they’ll immediately be sucked into the fuel filter. When the fuel filter gets clogged, no fuel will flow to the carburetor or to the primer bulb. You will need to drain your fuel tank and scrub it out well with a brush, rag and some clean gasoline. Also, pull off the fuel filter and replace it, which is less time-consuming than trying to clean the small screen.
Dirty Suction Hose
After the fuel leaves the tank, it goes into the suction hose. This rubber hose connects to the intake valve on the carburetor. When the crankcase rotates, the pressure generated pushes on a diaphragm in the carburetor, which sucks the fuel from the tank and into the carburetor. If the suction hose can’t deliver enough fuel to the carburetor, the primer bulb also won’t fill. The suction hose should be replaced every season to ensure the best results.
Dirty Return Hose
After the fuel is mixed and pumped into the cylinder for ignition, any leftover fuel is dumped from the carburetor back into the fuel tank. The primer bulb is attached to this return line, completing the fuel cycle. If the hose is dirty or cracked, fuel will start leaking out somewhere else in the fuel system. These hoses and the bulb itself tend to get loose over time, and any bad connection will also cause the primer bulb to stop filling. Return hoses and primer bulbs should be replaced on a bi-seasonal basis at a minimum, but for the best operation they should be replaced at the same time as the main suction hose.
Dirty or blocked jets will stop any movement of fuel in the system. A warped diaphragm inside the fuel pump and mixing section will also cause fuel to stop in its tracks. The reason is due to the pulsing of the crankcase and the pressure generated: if the carburetor doesn’t respond properly to the pulsing, the fuel will stop, and the primer bulb won’t fill. Carburetor problems should be taken care of by a service mechanic.