We never think about the fuel pumps in our cars. They sit there year after year, pumping gas to the carburetor, never requiring repair or adjustment, until they eventually fail. They are so reliable that we don't carry spares with us, nor do we consider replacing them at 30, 40, 50-thousand mile intervals.
How a mechanical (that's what is in most of our collector cars) fuel pump works is deceptively simple. It consists of a rubber diaphragm that is sandwiched between two halves of the housing and actuated by some sort of arm. More about that later, but for now we must distinguish between "pusher" and "puller" pumps.
Mechanical fuel pumps are "puller" pumps. That is, they have strong suction ability, which is needed to pull gasoline from the tank and up to the engine. The distance from the pump to the carburetor is relatively short, so "puller" pumps don't have to work hard to keep up pressure in the fuel line.
"Pusher" pumps, on the other hand, are better at pushing fuel down the line than they are at drawing it up in the first place. Electric pumps (British sports cars, for instance, used these) are generally "pushers," so they are placed close to the tank and rely on a siphon to get fuel.
The camshaft has an eccentric that is designed to operate the pump's lever arm. The arm's fulcrum point is very close to the part that actuates the diaphragm, so very little movement of the arm at the camshaft results in significant movement of the diaphragm. Each 360-degree rotation of the camshaft results in one complete cycle of the diaphragm itself.
As the diaphragm is pushed up, gasoline above it is pressurized and moved out of the pump and into the fuel line to the carburetor. The pressure of the fuel is typically designed to be 2-6 psi, depending upon the manufacturer. As the lever arm allows the diaphragm to return to its rest position, one-way valves in the pump body prevent the return of fuel from the carburetor and open a path for fuel to be drawn from the gas tank. The vacuum created by the diaphragm's movement draws fuel into the chamber, helped by the siphon already present in the fuel system.
Many cars used dual stage pumps to help operated vacuum-actuated windshield wipers. The chief disadvantage of vacuum wipers was, without the assistance of the pumps, that under engine loads the intake manifold's vacuum went to zero and the wipers stopped momentarily. This problem was "cured" with dual stage pumps.
The fuel section of a dual stage pump functions in the same way as ordinary fuel pumps. However, the rotation of the camshaft eccentric in the vacuum pump also operates the vacuum booster section by actuating the pump arm, which pushes a link and the bellows diaphragm assembly upward, expelling air in the upper chamber through its exhaust valve out into the intake manifold. On the return stroke of the pump arm, the diaphragm spring moves the bellows diaphragm down, producing a suction in the vacuum chamber.
This suction opens the intake valve of the vacuum section and draws air through the inlet pipe from the windshield wipers. When the wipers are not operating, the intake manifold suction (vacuum) holds the diaphragm up against the diaphragm spring pressure so that the diaphragm does not function with every stroke of the pump arm. When the vacuum is greater than the suction produced by the pump, the air flows from the windshield wiper through the inlet valve and vacuum chamber of the pump and out the exhaust valve outlet to the manifold, leaving the vacuum section inoperative.
With high suction in the intake manifold, the operation of the wiper will be the same as if the pump were not installed. When the suction is low, as when the engine is accelerated or operating at high speed, the suction of the pump is greater than that in the manifold and the vacuum section operates the wipers at a constant speed.
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