Fuel Pump - Mechanical
Mechanical fuel pumps are used in older engines which are equipped with a carburetor. This type of fuel pump uses the power of the engine via a lever which is connected to the crankshaft. This type of pump is used on systems which do not have a battery or some other form of electrical energy. The main purpose of a mechanical fuel pump, is to siphon fuel from the gas tank and push it into the fuel bowl in the carburetor. This type of pump is normally bolted to the engine block or head.
The main component of a mechanical fuel pump is the diaphragm, which is a piece of rubber which gets pushed up and down in the pump. When this flexible diaphragm is pulled to the bottom of the pump, volume inside the chamber increases, which causes pressure to decrease. When the pressure decreases, fuel is pushed into the pump from the tank. As the diaphragm is forced to the top of the chamber using a diaphragm spring, fuel in the pump chamber is squeezed through the outlet port and into the carburetors fuel bowl.
As the fuel level in the float bowl exceeds a certain level, the inlet valve which is connected to the carburetor closes. This prevents the fuel pump from pumping more fuel into the carburetor,than it needs or can handle. The diaphragm continues to allow pressure to the diaphragm, even though there is nowhere for the fuel to go. During the subsequent rotation, the eccentric pulls the diaphragm back to bottom of the chamber, where it remains until the inlet valve to the carburetor opens again. Having pressure build up for a while is not a big deal in a mechanical fuel pump, because it does not generate much more than 10-15 psi.
There are some common problems that mechanical fuel pumps encounter. The most common issue is when the diaphragm cracks, which may cause leaks in the crankcase and a drop in fuel pressure. Low pressure between the fuel pump and the fuel tank, combined with the heat of the engine, may cause fuel to vaporize in the supply line. This can cause fuel starvation, which can cause the engine to stall. An advantage of a mechanical fuel pump is that they are much safer because of their low operating pressures, and because they will turn off when the engine stops running.
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