Volkswagen Air-Cooled 1970-1981 Repair Guide

Understanding the Fuel System


An automotive fuel system consists of everything between the fuel tank and the carburetor. This includes the tank itself, all the lines, one or more fuel filters, a fuel pump (mechanical or electric), and the carburetor.

With the exception of the carburetor, the fuel system is quite simple in operation. Fuel is drawn from the tank through the fuel line by the fuel filter, and from there to the carburetor where it is distributed to the cylinders.


Normally, fuel tanks are located at the rear of the vehicle, although on most rear-engined cars, they are located at the front. The tank itself also contains a fuel gauge sending unit, and a filler tube. In most tanks, there is also a screen of some sort in the bottom of the tank near the pick-up to filter out impurities. Since the advent of emission controls, tanks are equipped with a control system to prevent fuel vapor from being discharged into the atmosphere. A vent line in the tank which is connected to a filter in the engine compartment. Vapors from the tank are trapped in the filter canister, where they are routed back to the fuel tank, making the system a closed loop. All the fumes are prevented from escaping to the atmosphere. These systems also require the use of a special gas cap which makes an airtight seal.


There are two types of fuel pumps in general use: the mechanical pump and the electric pump. On carbureted engines, mechanical pumps are the more common of the two. Electric pumps are used on all fuel injected cars (and some carburetor-equipped cars) in addition to seeing wide use on a number of imported cars.

Mechanical fuel pumps are usually mounted on the side of the engine block and operated by an eccentric lobe on the engine's camshaft. A pump rocker arm rests against the camshaft eccentric and as the camshaft rotates, causes the rocker arm to rock back and forth. Inside the fuel pump, the rocker arm is connected to a flexible diaphragm. A spring is mounted under the diaphragm to maintain pressure on the diaphragm. As the rocker arm rocks, it pulls the diaphragm down and then releases it. Once the diaphragm is released, the spring pushes it back up. This continual diaphragm motion causes a partial vacuum and pressure in the space above the diaphragm. The vacuum sucks the fuel from the tank and the pressure pushes it toward the carburetor.

As a general rule, mechanical fuel pumps are quite dependable. When trouble does occur, it is usually caused by a cracked or broken diaphragm, which will not draw sufficient fuel. Occasionally, the pump arm or spring will become so worn that the fuel pump can no longer produce an adequate supply of fuel, but this condition can be easily checked. Older fuel pumps are rebuildable, but late-model pumps have a crimped edge and must be replaced if defective.

There are two general types of electric fuel pumps in use today. The impeller type pump uses a vane or impeller which is driven by an electric motor. These pumps are often mounted in the fuel tank, though they are sometimes found below or beside the tank.

The bellows-type pump, is becoming rare. The bellows pump ordinarily is mounted in the engine compartment and contains a flexible metal bellows operated by an electromagnet.

Most electric fuel pumps are not rebuildable and if defective must be replaced. Minor service is usually confined to checking electrical connections and checking for a blown fuse.


In addition to the screen located in the bottom of the fuel tank, all fuel systems have at least one other filter located somewhere between the fuel tank and the carburetor. On some models, the filter is part of the fuel pump itself, on others it is located in the fuel line, and still others locate the filter in the carburetor inlet or the carburetor body itself.

The fuel filter is usually a paper or bronze element which screens out impurities in the fuel, before it has a chance to reach the carburetor. If you replace the fuel filter, you'll be amazed at the bits of sediment and dirt trapped by the filter.


The carburetor is the most complex part of the entire fuel system. Carburetors vary greatly in construction, but they all operate basically the same way; their job is to supply the correct mixture of fuel and air to the engine in response to varying conditions.

Despite their complexity in operation, carburetors function because of a simple physical principle-the venturi principle. Air is drawn into the engine by the pumping action of the pistons. As the air enters the top of the carburetor, it passes through a venturi, which is nothing more than a restriction in the throttle bore. The air speeds up as it passes through the venturi, causing a slight drop in pressure. This pressure drop pulls fuel from the float bowl through a nozzle into the throttle bore, where it mixes with the air and forms a fine mist, which is distributed to the cylinders through the intake manifold.

There are six different systems (fuel/air circuits) in a carburetor that make it work; the Flat system, Main Metering system, Idle and Low-Speed system, Accelerator Pump system, Power system, and the Choke system. The way these systems are arranged in the carburetor determines the carburetor's size and shape.

It's hard to believe that the little single-barrel carburetor used on 4 or 6 cylinder engines have all the same basic systems as the enormous 4-barrel's used on V8 engines. Of course, the 4-barrels have more throttle bores ("barrels") and a lot of other hardware you won't find on the little single-barrels. But basically, all carburetors are similar, and if you understand a simple single-barrel, you can use that knowledge to understand a 4-barrel. If you'll study the explanations of the various systems on this page, you'll discover that carburetors aren't as tricky as you thought they were. In fact, they're fairly simple, considering the job they have to do.

It's important to remember that carburetors seldom give trouble during normal operation. Other than changing the fuel and air filters and making sure the idle speed and mixture are OK at every tune-up, there's not much maintenance you can perform on the average carburetor.