Fuel Pump Overhaul

By Stephen Kassis

Early Chevrolet six cylinder fuel pumps are easy to rebuild and rebuild kits are available for most of the bolt-together pumps. If properly operating, original mechanical fuel pumps are more than adequate to supply fuel for six cylinder engines. Some prefer to have an electric fuel pump inline to help with fuel delivery. This will help with quicker startups when vehicles sit for long periods. However, we do not recommend using an electric pump exclusively to run the engine as it can cause excess fuel delivery to the carburetor.

1929-1933 fuel pumps are one of the easiest to rebuild since the diaphragm is simply bolted in place. After 1933, diaphragms had a fixed shaft with a slotted opening to hook onto the operating lever. It should be noted here that 1934 fuel pumps had a metal cover instead of a glass bowl. This was only done in that one year. References to glass bowls in this article should be ignored on 1934 pumps.

Alcohol has been added to most of our gasoline and fuel pumps are often the first part on the engine to suffer. Alcohol attacks the old style rubber diaphragm materials that were not alcohol resistant. This can cause fuel pump failure. If you have purchased a New Old Stock (NOS) fuel pump, it is recommended that the diaphragm is replaced before attempting to use it.

Another problem is that the new alcohol gas tends to break down faster than non-alcohol gas. This can cause problems due to the gummy substance that is formed when the fuel breaks down. It can plug fuel lines, tanks, fuel pumps and carburetors. In addition, it can cause intake & exhaust valves to stick, which in turn causes bent push rods and other problems. Whenever possible, it is highly recommended to run non-ethanol fuel in your vintage cars & trucks. This is especially important when the vehicle will be inactive for long periods.

Fortunately, fuel pumps on early Chevrolet 6-cylinders are easy to rebuild. If the fuel pump fails to pump fuel, don’t automatically assume that the fuel pump is the problem. The first thing to check is to see that fuel is getting to the fuel pump. Is there fuel in the tank? Sometimes the fuel gauge may not read properly or there is something plugging the fuel line. The pump may be fine but cannot pump fuel because fuel is not getting to the pump.

If there is a good amount of fuel in the tank, fuel should gravity feed from the tank to the fuel pump connection when the line is lowered below the level of the tank. Remove the inlet fuel line from the pump and push the line down to see if fuel will gravity feed. If no fuel will flow with this test, remove the gas cap and have someone listen a few feet away from the opening while pressurizing the fuel line with compressed air in short bursts. CAUTION: fuel may gush out of the filler neck when doing this test, depending on how much fuel is in the tank. Keep everyone a few feet away from the opening. If air bubbles are heard when sending compressed air back through the line, then there may have been some foreign matter that is in the tank plugging the fuel outlet. Recheck for gravity flow of fuel.

It is unusual but possible that the fuel pump lobe on the camshaft is worn to the point the fuel pump lever will not operate properly. This is a more serious problem and could require replacing the camshaft. To test the fuel pump, remove the fuel line that goes to the carburetor. Remove the two bolts that hold the fuel pump to the block. This will allow the fuel pump to be removed from the block and pumped manually. Reattach the tank line to the inlet of the pump and pump manually. If no fuel will pump manually, remove the fuel pump from tank feed line and move to the work bench.

If fuel is gravity feeding to the engine compartment, then further testing of the fuel pump is necessary. Place the inlet side of the pump into a pan of water and manually operate the pump lever. If water is not pumped out with this manual method, check the seal on the glass bowl. If the seal is not good here, the pump cannot create a vacuum to draw fuel from the tank. TIP: A light coat of grease or petroleum jelly on the gasket will help the glass bowl seal to the gasket. Re-check and if still no suction, it will indicate that the pump should be rebuilt.

The first step is to determine if you have an original AC fuel pump on your vehicle. Original fuel pumps on Chevrolets were made by AC. Original AC fuel pumps have raised letter “AC” cast into the pump. There were many manufacturers of replacement fuel pumps for Chevrolet engines. Some were marked “BC” or had no markings at all. If you have an aftermarket fuel pump (not AC), be aware that original rebuild kits may not work on your pump as internal parts can be different. However, even AC changed designs over the years and made replacement pumps a little different than the original.

If you have an original AC pump, obtain a fuel pump rebuilding kit. It should contain: New valves, alcohol resistant diaphragm, cam springs, cam cover gasket, glass bowl gasket and fuel pump to block gasket. Other parts may be included, but these are the minimum that should be in the kit. Drain off any gas remaining in the fuel pump. Remove the glass bowl & wire bail that holds it in place. Remove the 6 screws that hold the top and bottom parts of the pump together. Separate the top and bottom halves. Sometimes, these can be stubborn and will require separating with a putty knife and hammer. Be cautious if this is necessary, so there is no damage to the pot metal pump housings. The top half contains the valves. On early pumps (1929-1936) these valves are inside of a brass hex fitting. Remove the fittings carefully as there is a fine spring inside each one. Remove the hex shaped, flat Bakelite valves.

On 1937 and later fuel pumps, self-contained valve units replace the earlier design. These valves are removed by removing the valve retainer and screws. The lower half of the pump differs in design depending on the year of the pump. Early pumps (1929-1933) have a diaphragm that is held with a simple nut, flat washer & lock washer. These are easily removed. From 1934 and later, the diaphragm is fixed onto a shaft. This type of diaphragm must be replaced as a unit.

Drive the lever arm pivot pin out sideways. CAUTION: Some pump housings have a tapered pin and must be driven out the large end or damage to the housing may result. Some later shafts have “C” clips locking the shaft in place. Once the pump has been completely disassembled, clean all the parts in solvent.

With all parts clean and ready for assembly, start with the lower half of the pump. Inspect the lever arm for damage or wear. If badly worn, a new arm should be installed. Install the new lever springs and caps. Install a new gasket on the lower pivot shaft cover (1929-1933 pumps only). Install new diaphragm making sure that the holes in the diaphragm match up to the lower housing holes.

On 1929-1936 pumps, inspect the brass valve seat for pitting. If this surface is rough, the new hex shaped valves may not seal properly. This would prevent proper operation of the pump. You can easily make a valve seat tool to smooth out the seat. Get a round steel shaft that closely matches the inside diameter of the valve bore. On one end, drill and tap a small hole for a small machine screw. Use a small Dremel sanding disk attached to the end. Slide the tool into the bore and manually spin this to smooth out the valve seat. Clean out the bore of any residue and you are ready for new valves.

Install new valves in the upper half. On 1929-1936 pumps, a drop of light oil on the Bakelite valve is recommended. This will help insure good operation until fuel gets to the pump. Leaning the pump on its side, install the spring and valve caps with new gaskets.

On 1937 and later pumps, the inlet valve has the spring facing downward and outlet should have the spring facing upward. Install the valve retainer with convex side up and a new gasket for the glass bowl. When installing the glass bowl, do not over-tighten the bail as it may damage the die cast pump housing. Assemble the top and bottom halves of the pump together with the 6 screws. Insure that the housings are matched properly and diaphragm is in alignment before screwing them together. Start all 6 screws and tighten them evenly.

With everything assembled, it is time to test the rebuilt pump. First try pumping the lever while holding a finger over the inlet. Suction should be felt here. A good strong suction will indicate the pump is ready to install. Another test is to put the inlet side into a pan of water and pump the lever arm manually. You should see the glass bowl fill quickly and, once full, water should start shooting out the outlet side. If everything is working properly, a stream of 4 to 8 feet should come out of the outlet.

If the fuel bowl fails to fill, check the seal on the glass bowl. If the glass bowl gasket is not sealed, the pump will only pull air. If everything tests out correctly, remove the glass bowl and blow out any excess water. This would be an excellent time to install a paper element filter inside the glass bowl (1929-1936 pumps only). Paper element filters do a much better job of removing particles from fuel than the original screens used in these pumps. We strongly recommend the use of this type of filter, unless you already have a separate inline filter.

Inline glass bowl filters with replaceable paper element filters are available for later glass bowl pumps that only have a screen filter inside.