The Fuel System of Old Cars

The Fuel System of Classic, Vintage or Old School Cars


Purpose of Fuel System

The automotive fuel system has the job of supplying a correctly proportioned mixture of gasoline and air to the engine.

This mixture must be varied to suit the requirements of the engine under all operating conditions.  During starting, especially in cold weather, when the engine is being cranked, more fuel must be added to the mixture.

Gasoline vaporizes slowly in cold weather; therefore, the mixture must be enriched when starting a cold engine.  Under such conditions, the mixture of air/fuel ratio is about 9 pounds of air to 1 pound of gasoline.

After the engine has been warmed up, the proportion of gasoline in this mixture is reduced to about a 15-17 to 1 ratio.   This is about the engine’s most economical operating range.

However, during acceleration or high speed driving, there is a demand for more power, which requires a richer mixture, reducing the ratio to about 12 pounds of air for each pound of gasoline.

These varying conditions are what must be handled by the fuel system.


Fuel System Components

The fuel system consists of the fuel tank, fuel pump, filters, carburetor, intake manifold and fuel lines or tubes connecting the tank, pump and carburetor.

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The Fuel System of Classic, Vintage or Old School Cars

The Fuel System of Old Cars

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Fuel Tank

The tank for storing fuel is usually located at the rear of the vehicle.

This tank is of sheet metal construction and is held in position by supporting straps attached to the frame.

The filler opening of the tank is closed by a removable cap which must be properly vented.  The vent in the cap is so designated that air may enter the tank but no fuel can escape.

This prevents a vacuum from forming in the fuel tank as the fuel is being used.

The fuel line connection is usually located at or near the bottom of the fuel tank.

Most of the later model cars have a filtering element mounted at this fuel line connection.  In addition, the tank contains the sending unit of the fuel gauge.

Most tanks also have some sort of a vent pipe which allows air to escape when the tank is being filled.


Fuel Gauges

There are two basic types of fuel gauge that are used on most automobiles.

These are the balancing coil and thermostatic types.  Both types are similar in that each has a tank unit and a dash unit.

The tank unit has a float which moves up and down in the fuel tank with the fuel level, while the dash unit is simply a gauge which moves over a marked scale from empty to full.


The Fuel System of Old Cars

The Fuel System of Old Cars

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Balancing Coil

In the balancing coil system, the tank unit has a sliding contact that slides back and forth on an electrical resistance as the float moves up and down.

When the ignition system is turned on, this sends a variable electrical current to the dash unit which causes the gauge pointer to motor  back and forth indicating the fuel level in the tank.

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The thermostatic type also operates from a moving float except that the float lever bends a thermostatic blade which opens and closes the circuit by a set of contact points.

When the points are closed, a coil around the blade heats.  This causes the blades to open at the contact points and stops the current flow.

When the coil cools, the blade bends back, closing the circuit, causing the coil to heat until the pointer opens.  This sequence is repeated over and over.

The amount of bend in the blade is determined by the fuel level which in turn determines how hot the coil must get before the heat causes the blade to open the contacts.

The dash unit is in the same electrical circuit as the tank unit and when the contact points in the tank unit are closed, current is supplied to the coil in the dash unit.

This causes the thermostatic blade to bend which in turn moves the attached gauge pointer.

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Fuel Tank Lines and Filters

We mentioned earlier that a filter is usually placed in the fuel line connection to the fuel tank.

The reason being that water and dirt that accumulate in the fuel tank can cause a clogged or restricted fuel line or filter, malfunction of the vapor discharge valve, fuel pump or carburetor.

Condensation is the greatest source of water entering the fuel tank.

It forms from the moisture in the air.  When the moist air strikes the cold interior walls of the fuel tank, condensation takes place.

If accumulation of sediment in the rear tank filter is excessive, the fuel tank should be removed and flushed and the lines from the fuel pump to the tank should be blown out.  Most fuel pumps have a filter incorporated into the unit itself.

Also, in later model cars, a separate unit is usually incorporated into the fuel line between the pump and the carburetor.

In addition, carburetors often contain filter screens.  A restricted fuel tank vent can cause low fuel pump pressure and volume and may, in some instances, result in a collapsed inlet line hose or collapsed fuel tank because of the vacuum created in the fuel tank as the gasoline is used.

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Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.

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