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Old 08-19-2008, 10:38 PM
Don S's Avatar
Don S Don S is offline
Gone,But not Forgotten.
Join Date: Feb 06, 2002
Location: Burleson TX
Posts: 5,613

FUEL PUMP, ELECTRIC 101 update 02.10.05

There are many advantages of using an electric fuel pump on vehicles equipped with carburetors.
Among these are;

The carburetor float chamber can be primed ready for starting before the engine rotates.
The fuel delivery system can be checked without the engine running. After running out of gas the carburetor can be filled without running down the battery and overheating the starter.
The pump motors can be put on a hidden switch and used as a theft deterrent.
The pump will not leak gasoline into the crankcase.
Fuel can be removed from the tank by the pump.

One might think that an electric fuel pump is just another electric fuel pump but there is a reason they don’t all look alike or cost as much to manufacture and purchase. Many fall short when it comes to dependability and lasting qualities.

Putting pump types into categories might help us explain the attributes of each pump.
First lets look at causes a standard mechanical fuel pump to move the fuel. A rotating cam moves a plunger or lever that in turn moves a diaphragm. As the diaphragm moves it will reduce the volume in the liquid chamber and the fuel is forced out.
There are two spring loaded one way valves in the chamber. One valve prevents the fuel from returning to the tank (input) and the output valve (on the carburetor side) prevents the fuel from returning to the chamber. If ether valve is not operating with a good seal the efficiency of the pump will be greatly reduced.

A spring is used to pull the diaphragm back out of the liquid chamber and that draws the fuel from the tank. The spring also keeps the rod or lever on the cam. The cam, rod or levers and springs can wear out or break.
Many models of mechanical fuel pumps can leak fuel into the engine if the diaphragm is ruptured. Adding an electric pump to feed a mechanical pump can shorten the life of the older diaphragms by causing a rupture. The addition of an electrical pump that is not running will slightly impede the flow of fuel to the mechanical pumps.

There are two basic types of liquid chambers used in the design of electric fuel pumps. One is the similar the mechanical fuel pump described above and can be driven with an SCR, solenoid or electric motor. The other type of pump known as a gyrator is a rotating single cam wheel and driven by an electric motor. Gyrator pumps are used in fuel tanks of most modern vehicles.
Of the electric fuel pump models the gyrator pumps have the best long-term performance.

… The mounting of the electric fuel pumps should be done in accordance with the manufacture guidelines. Mounting them low out of harms way is recommended. In actual practice in the case of 4x4s mounting them in front of the tank can cause problems. As the truck ascends steep angles and requires more fuel for power the pump will be much higher than the fuel level in the tank and not operate at peak efficiencies. Mount the pumps as far as practical from heat sources. Using a filter between the tank and pump protects the pump but restricts the flow to the pump.
Use a rollover valve and switch for safety sake!

You have two things to look for when selecting an electric or mechanical fuel pump. These are fuel pressure and flow rate such as gallons per minute or hour. Also one must consider the conveance of an electric over a mechanical plus the longevity of the pump and the price.

The requirements for some FSJs are here;
A fuel pump for a six-cylinder engine should put out 4 to 5 psi and a fuel pump for an eight-cylinder engine should put out 5 to 6.5 psi. Both six and eight cylinder fuel pumps should pump one pint in 30 seconds at idle RPM. These are ‘specs’ for 1981 Jeeps per the AMC TSM.

The rating of pressure and volume for a pump is at the out put of the pump.
The fuel pump also has to overcome pressure loss in the fuel lines due to friction. The stated pressure/volume at the output of the pump is not what the pressure/volume is as delivered at the carburetor except … A pressure at rest is equal on all sides i.e. fuel not flowing.

My ‘76 Wagoneer with a 401cid Bored .030 with an MC-4350 or its MC-4300 carburetor requires at least an $80.00 pump and prefers the Holley Red Top at over $120.00.

Have a good one while you can still laugh about it.. Don S..
I used to be quite indecisive but maybe now days I’m not so sure!

Sold our 1976 Wagoneer 406, MC4300, TH400, QT, TruTrac, 2" lift, 31x10.50s, duel Optimas,
It’s took us over 161 Colorado Mountain Passes, 3 Jeep Jamboree USAs & 2 Ouray Invasions from 1985 to 2010
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