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Fuel Pump Overhaul

By Stephen Kassis

The fuel pump on early Chevrolet 6-Cylinder models is a very simple part. Rebuilding these pumps is equally simple. It is important to do some initial troubleshooting to make sure that the fuel pump is the actual problem. Original mechanical fuel pumps should be more than adequate for operating vintage engines. Supplemental electric fuel pumps are unnecessary if the original mechanical pump is operating properly.

It is common to assume that the fuel pump is bad when fuel delivery is a problem. However, other things can cause the same symptoms and should be checked before doing a fuel pump overhaul. Make sure there is plenty of gas in the gas tank. Yes, this would seem rather basic, but it has happened many times that the assumption of gas in the tank has led to countless hours of unnecessary troubleshooting.

Another simple fix is to check the gasket on the fuel sediment bowl. If this gasket is not sealing properly, air will be allowed in instead of creating a vacuum. We recommend using a neoprene or cork/rubber gasket for the glass bowl. Also a light coating of grease on the gasket will help to seal it up.

If the tank is low on fuel, sometimes sediment in the tank will cause the pickup tube in the fuel tank to be clogged. Blowing back through the fuel line will temporarily clear up this problem, but it is only a temporary fix. A similar symptom can occur when a gas tank has been sealed after cleaning. The sealer gets into the pickup tube and eventually pulls away from the inside of the tube, collapses and blocks the outlet. If this problem persists, dropping the tank for a good cleaning and inspection will be necessary.

When old cars have been sitting for long periods, gas in the tank, fuel lines or a carburetor can turn into varnish. Modern fuels, with alcohol added, can have this occur in a much shorter time than previous non-alcohol fuels. When this happens, fuel lines, fuel pumps and carburetors can become clogged with the substance. The resolution in this case is to remove the gas tank for cleaning, flush or replace all fuel lines and disassemble the carburetor for inspection or rebuilding. It is recommended that a fuel stabilizer is used when storing your old car for periods of more than 2 months. This will delay the breakdown of fuel in the fuel system of the vehicle while in storage.

For small areas that have varnish, an aerosol cleaner such as B-12 Chemtool carburetor cleaner is an excellent way to remove varnish.

If it is determined that the gas tank is clean and fuel lines are clear, then it is time to test and/or rebuild the fuel pump. Remove the fuel lines from the pump and remove the pump from the block. Manually operate the fuel pump lever while holding a finger over the inlet. There should be good suction at the inlet. If this quick test fails or is weak, then move to the workbench. Remove the screws that hold the top section of the pump to the bottom section. Separate the top and bottom sections to expose the fuel pump diaphragm. Alcohol in modern fuels can attack original fuel pump diaphragms. Look for areas on the diaphragm that look like the rubber has been eaten away and cloth mesh is exposed. This will indicate a bad diaphragm and require replacing it. Fuel pump kits should include an alcohol resistant fuel pump diaphragm, new valves, springs, pivot, fuel bowl, bottom cover and pump mounting gaskets. Clean all old parts to be re-used thoroughly before assembly.

Early fuel pumps from 1929-1936 have valves that are made up of a fine hair-spring along with a Bakelite wafer to open and close the valve. After 1937 the valves were a self-contained disc that is pressed into the pump housing. It is important to inspect the valve seating surface on the early pumps to ensure that the surface, inside the valve bore, is smooth. If rough, it will allow the valve to leak. It is possible to make a tool (similar to a Dremel sanding disc) to smooth the valve seating surface by hand, if necessary.

Put a small drop of oil on each new valve wafer. This will lubricate the valve to insure it will operate until fuel gets up to the pump and lubricates the valves. Install the new Bakelite valve into the bore and make sure it is seated properly. Insert the new valve spring into the valve cap and carefully install the valve cap into the pump housing. Use caution to make sure the valve spring and valve wafer are properly installed. These procedures are not needed on 1937 and newer pumps as the valves were a self-contained unit.

The diaphragm on 1929-1933 pumps is easy to replace. Unscrew nut in the center, remove the lock washer and flat hex washer, and large flanged washer. Remove and discard the old diaphragm. Reverse the procedure with the new diaphragm, making sure to align the screw holes in the diaphragm with the holes in the housing. It is a good idea to carry a spare diaphragm in the vehicle as these are easily changed on the side of the road.

Though the two heavy springs on the lower part of the pump seldom need replacing, the kits usually have these and we recommend that they be replaced when rebuilding the pump. Remove the lower cover by removing the three screws holding it. This will expose the springs. Remove the pivot shaft and remove and replace the springs and pivot pin. This procedure will be required on later pumps in order to replace the fuel pump diaphragm.

When the diaphragm, valves and springs have been replaced, install the lower cover & gasket. Reassemble the top of the pump to the bottom. Tighten evenly being careful not to over-tighten, which would cause the housing to warp. An option for most glass bowl fuel pumps is to install a paper element filter inside the glass bowl. Paper element filters are a much more efficient method of filtering than the glass sediment bowl with screen.

To test the rebuilt pump, operate the lever arm manually while holding a finger over the fuel inlet. Strong suction should be felt. Another way to test would be to place the inlet fitting in a pan of water and operate the lever arm manually. The fuel bowl should fill in about 3-4 pumps if the fuel pump is operating properly. Additionally, it should shoot water out of the outlet at least 10 feet or more. Note: If you have one installed, remove the paper element filter before attempting this test. Also, be sure to dry out the water before installing the pump.

A common leaking area is the gasket seal on the glass bowl. This seal must be good in order for the pump to operate. If the glass bowl was over-tightened at some time in the past, it may have warped the housing. Because the housing is pot metal, you must be very careful when attempting to straighten it. If severe, the warping prevents sealing and must be corrected. It is possible to correct the warping by placing the upper pump housing on a flat surface. Use two small C-clamps to apply light force to the raised ends. This is best done in stages, lightly tightening the clamps every few days until the warping is removed. Tightening too fast can break the delicate pot metal. After straightening, check to make sure the fuel bowl has a good seal. 

Once the pump is working properly, hook up the inlet fuel line to the pump. Manually operate the lever on the pump until the glass bowl is full and fuel starts to come out of the pump outlet. Install a new gasket and bolt the fuel pump to the block. Hook up the outlet line to the carburetor. Start the engine and check for leaks.

One final note - With engines that are 60 to 80 years old, the fuel pump may have been replaced with an aftermarket pump. If your pump does not have the "AC" markings, it may not be an original designed fuel pump. Check this before ordering a fuel pump kit or you may be disappointed to find that none of the parts will work.

Related Parts

FS-371 - 1929-1933 Fuel Pump Diaphragm

RW-717 - 1916-1960 Carburetor & Fuel Pump Fitting

FS-607 - Glass Bowl Gas Filter Assembly

FS-607AC - "AC" Glass Bowl Gas Filter Assembly

FS-235 - 1929-1933 Glass Bowl Fuel Pump Paper Filter

FG-4 - 1952-1957 Glass Bowl Fuel Pump Paper Filter

AF-429 - STA-BIL Fuel Stabilizer 4 OZ

AF-429B - STA-BIL Fuel Stabilizer 32 OZ

AF-3 - B-12 Chemtool 16 OZ Aerosol

PA-67 - POR-15 Fuel Tank Repair Kit

H-19 - Fuel Tank Repair Kit

PA-61 - POR-15 Gas Tank Sealer - Quart

H-3A - Alcohol Resistant Gas Tank Sealer