Fuel Pumps How It Works for Classic Cars

This article is all about the Fuel Pumps How It Works for Classic Cars

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Thus far, we have a fuel supply and a means of removing some of the impurities which will probably accumulate.   Now, though, we have to get the fuel to the carburetor.

Obviously, the fuel pump serves the purpose; but how does it operate ?  Where does it get its power from?  Also, automobile engines don’t run at a constant speed; so, therefore, the pump must supply variable a mounts of fuel Right ?

The pump must then have a variable output, dependent upon the engine demand for fuel.

Fuel pumps are usually mounted on the side of the cylinder block or between the two cylinder banks on some of the V-8 engines.

Fuel Pumps

Fuel pumps on in-line engines which are mechanically operated are actuated by means of the fuel pump rocker arm and an eccentric on the cam shaft.

On V-8 engines which have the fuel pump mounted between the two banks of cylinders, the rocker arm rests on a push rod.  The lower end of the push rod rides on the cam shaft eccentric.

The rotating eccentric provides the power to the rocker arm which provides the pumping action.

Fuel Pumps How It Works for Classic Cars

Fuel Pumps How It Works for Classic Cars

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Other major components of the fuel pump are: 

A flexible diaphragm mounted between the upper and lower fuel pump housing and attached to the end of the rocker arm assembly inside the fuel pump.

The inlet and outlet valves  and springs, gaskets and seals On most models, a fuel filter.

As the cam shaft rotates, the eccentric causes the rocker arm to rock back and forth.

The inner end of the rocker arm is linked to a flexible diaphragm which is clamped between the upper and lower pump housings.

On the fuel intake stroke, the cam shaft eccentric causes the rocker arm to force the fuel pump diaphragm to move against the diaphragm spring pressure.

The moving diaphragm draws fuel through the intake valve into the pump chamber and automatically closes the outlet valve.  Fuel is drawn from the fuel tank through the fuel intake line to replace the fuel drawn into the chamber.


When the rocker arm moves the diaphragm thus enlarging the chamber, the pressure on the fuel at the pump intake valve is reduced because the movement of the diaphragm tries to form a partial vacuum.


That is, it is less than the pressure exerted on the fuel in the tank.  Actually, then, it is the air pressure on the fuel in the tank which forces the fuel into the pump chamber through the pump inlet valve because of the greater pressure on the fuel in the tank.

If the tank vent were blocked, there would be no pressure difference between the sides of the pump inlet valve when the rocker arm forces the diaphragm to enlarge the chamber and the fuel tank.  A partial vacuum would be created in the fuel tank and no fuel would flow.

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All fuel drawn into the chamber must first pass through a filter bowl and screen in the line that is usually located on the top or bottom of the fuel pump.

As the cam shaft eccentric continues to rotate, the rocker arm relieves the pressure on the diaphragm spring and allows the spring to push the diaphragm back to its original position which reduces the size of the pump chamber.

This action increases pressure on the fuel in the chamber, closing the inlet valve to prevent fuel flowing back to the tank and opening the outlet valve.  The fuel is thus forced through the pump outlet to the carburetor.

Fuel is delivered to the carburetor only when the fuel inlet valve in the carburetor is open.

The inlet valve in the carburetor float chamber is closed by pressure of the fuel on the float when the specified fuel level in the float chamber is reached.

It is the float level in the carburetor which regulates the demand on the fuel pump.  When the carburetor float chamber begins to empty, the inlet valve opens and accepts more fuel from the pump.


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Let’s take a closer look at the fuel pump and see why it doesn’t simply supply a constant flow of fuel to the carburetor. 

The fact that the rocker arm forces the diaphragm to enlarge the pump fuel chamber on the intake stroke was stated earlier.   Actually, the rocker arm serves only it lift the diaphragm against the diaphragm spring pressure.

When there is no demand for fuel from the carburetor, the diaphragm spring tension is not strong enough to force the diaphragm downward against the pressure built up in the inlet chamber of the pump because the carburetor intake valve is closed by the float.

Thus, the up and down rocker action continues, but the diaphragm remains stationary until pressure against the carburetor float is relieved by demand for fuel at the carburetor.

Pressure leak down bleed holes are incorporated in the intake and outlet valve assemblies of all fuel pumps.  Also, an air vent is located in the fuel pump body which relieves the air pressure built up on the spring side of the diaphragm.

Related Articles – Float System of Classic or Old Cars Carburetor 


Fuel Pumps How It Works for Classic Cars  Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.