Mechanical fuel pumps are used on older engines that have carburetors (though some may have a low pressure electric fuel pump mounted in or near the gas tank). The pump siphons fuel from the gas tank and pushes it to the carburetor when the engine is cranking or running.
Mechanical fuel pumps use a lever that rides on the camshaft to pump a rubber diaphragm inside the pump up and down. This creates suction that pulls fuel into the pump, and then pushes it along. A pair of one-way valves inside the pump only allow the gas to move in one direction (toward the engine).
The output pressure of a mechanical fuel pump is typically quite low: only 4 to 10 psi. But little pressure is needed to keep a carburetor supplied with fuel.
A leak in the diaphragm or one-way valve inside a mechanical fuel pump will cause a loss of fuel pressure and starve the carburetor for fuel. This may cause the engine to run lean, misfire, hesitate or stall. If the pump fails entirely, no fuel will be delivered to the carburetor and the engine will not start or run.
Fuel leaks are another common problem, usually due to cracks or holes in the rubber diaphragm, or loose inlet or outlet fittings.
A mechanical pump can be checked any one of four ways:
Remove the air cleaner, look into the throat of the carburetor and pump the throttle linkage. You should see fuel squirt into the carburetor if the pump is working. If you do not see any fuel squirting into the carburetor, the fuel pump has probably failed (or the fuel line or fuel filter are blocked, or the tank is out of gas).
Visually inspect the pump. If you see any fuel dripping from the pump, the diaphragm inside has failed and the pump needs to be replaced.
WARNING: A leaky fuel pump is dangerous because the fuel may ignite and start a fire!
Another way to check the pump is to disconnect the fuel line at the carburetor and place the end of the line into a container. Crank the engine to see if the pump is pushing any fuel through the line. Strong steady spurts of fuel mean the pump is working. No fuel or a weak stream means a bad pump, a plugged fuel filter, fuel line blockage or no fuel in the tank.
WARNING: Do not smoke near gasoline, and do not allow any sparks near the carburetor or open fuel line as this may ignite the fuel causing a fire! Do not spill gasoline on a hot engine. Wait until the engine has cooled to work on the fuel system. Also, avoid skin contact with gasoline and do not breathe the vapors.
You should also check fuel pump pressure. Connect a fuel pressure gauge to the pump outlet, or tee a gauge into the fuel line at the carburetor. Crank the engine and note the pressure reading on the gauge. If there is no pressure, or if pressure is less than specifications, the pump is bad and needs to be replaced.
If the pump needs to be replaced, disconnect the fuel inlet and outline lines from the pump. Plug the inlet fuel line so fuel does not leak out (the line is lower than the fuel tank and will dribble fuel if it is not plugged).
Unbolt the fuel pump from the engine. There are usually two bolts. Once the bolts have been removed the pump should come off.
Remove the old gasket from the pump mounting surface on the engine, and clean the mounting surface. Do not allow gasket residue to fall inside the engine opening.
Install a new gasket on the pump. Apply gasket sealer to both sides of the gasket for a leak-free seal. Apply a dab of grease on the tip of the pump lever where it contacts the camshaft, then mount the pump on the engine. Make sure the pump lever is correctly aligned with the lobe on the camshaft, otherwise the pump lever may be damaged when the engine is cranked.
Reconnect the fuel inlet and outlet lines, being careful not to over-tighten or damage the4 fittings.
Crank the engine and check for leaks. If the pump was installed correctly and there are no other problems (like a plugged fuel filter), the engine should start and run normally.
The fuel filter should also be replaced, and if the old filter is plugged it may be necessary to drain and clean the gas tank, or to replace the gas tank if it is rusty inside.