Fuel pump rebuilding for amateurs

As most of you know, one of the few weak points of our beloved old flatheads is the mechanical fuel pump. Because of this, probably 80 percent (or more) have auxiliary electric fuel pumps installed. Many articles have been written by many authors (including this one) about electric fuel pump installation.

Therefore, it is about time to talk about rebuilding these original mechanical fuel pumps. After all, these pumps are dual action with the top portion dedicated to pumping fuel, while the lower portion is a vacuum pump to provide power to the windshield wipers and vacuum antenna (if so equipped).

Although the specifics will refer to the flathead fuel pumps, the general procedure will apply to most fuel pumps (and even some other components). Even amateurs can do it following these methods.

The best place to start is to have a new rebuilding kit on hand. Set up a card table and cover it with butcher paper, masking paper or even paper towels. Open up the kit and line up all the pieces on the top of the table neatly and study each of the pieces as you lay them out. You will probably find more pieces than you will eventually use, because most kits are assembled to rebuild more than one model of fuel pump.

If you don’t already have the fuel pump off the car, now is the time to take it off. Begin by removing all four lines from the pump, keeping a dry rag on hand to soak up the fuel that empties out of the lines. Remove the two mounting bolts, keeping your rag handy as the pressure is taken off of the cam, causing one last pump and spurt of fuel. Go outside to take off the glass bowl and filter, and bale and drain out any remaining fuel. The whole pump should be soaked in solvent, the exterior cleaned with a brush and then dried off.

You are now ready to start dismantling the pump itself. There are basically three sections to the pump: The fuel pump (upper section), the vacuum pump (lower section) and the center body or mechanical section. This assembly is held together by a total of 20 #10 fillister head machine screws about 5/8 inch long.

Using a file or hacksaw blade, cut a small groove in the edges of the bodies where the diaphragms are seen. These will guide you when reassembling for proper alignment. I suggest you cut one groove between the upper section and the body and two grooves between the body and lower section to avoid any later confusion.

Be careful taking out these screws, as they are under spring pressure and may go flying when the seal breaks. Start with the top fuel section, leaving two opposite screws only about halfway out. If the two sections are bound together by the diaphragm, tap the upper section with a small rubber or brass hammer until they separate and then remove the last two screws. Set the top section down on the table and draw a rough outline on the paper, and pile the screws beside the filter, glass bowl and bale in the same row.

Now remove the lower section using the same procedure, being careful of the very strong spring that will be coming loose. Set these parts on the paper in the same order you remove them and mark the paper with an outline of each piece. Observe how the diaphragms are connected to the operating arms and remove them, placing them on the paper in their respective rows.

Starting with the upper section, completely disassemble that section, lining up all the parts in the order they come off with drawings on the paper, so you can replace them in the right order after cleaning each part. Take out both reed valves and the gaskets underneath them. Follow the same procedure with the lower section, lining up the parts in a neat row.

Time now to dismantle the center section. Observe carefully how the arms and hooks are assembled. Start by removing the anchor pin that holds the pump arm onto the body. This may be riveted over and require grinding off the end in order to drive out the pin. After the pin is out, remove the arm assembly and small spring from the body observing the exact placement of this assembly. Take it apart and line it up on the paper, outlining each piece as you take it off.

You should now have four rows of parts. The first row is all the new parts from the kit. The second will be the upper fuel section. The third will be the lower vacuum section and the fourth row the center mechanical section.

With all these parts lined up, start replacing all the old parts on the paper, in the right order, with all the new parts that came in the kit. Put the old parts in a separate tray and mark the paper showing which parts are new. Now take the old metal parts that are being reused and soak these in carburetor cleaner, cleaning them thoroughly. Remember to clean off the carburetor cleaner by cleaning these parts with solvent or kerosene and dry them well.

You may want to lightly sandblast some of these parts and steel wool them for cosmetic reasons, but be sure to blow out all traces of any abrasives and steel wool, and lightly oil all metal interior parts. As each piece is cleaned to your satisfaction, replace them on the paper in the right order. Now aren’t you glad you made those drawings, and can put everything in the right order?

It’s time now to start reassembling your fuel pump in the exact reverse order that you took it apart. Start with the center section. I suggest you highly polish the cam face on the lever arm for obvious reasons. A Phillips head screwdriver will help you line up the cam arm and connecting parts for installation into the body against the tension of the lever return spring.

Once this section is assembled and working right, you can start putting together the lower section and then the upper section. The reed valves can go in only one way and I suggest you put a light coat of oil or grease on the small gaskets under the reed valves.

Once the three sections are assembled, you can put in the diaphragm for the lower section into the main body. The two diaphragms are different than each other and will only go in one way. Using the groove marks you put in earlier, line up the lower section to the main body with the spring in place. Use a couple of nails to line up the screw holes and install two opposite screws tightening about three-quarters of the way down.

It’s time now to install the upper diaphragm and spring using the same procedure. After you make sure both diaphragm rods are securely hooked, you can install and tighten all of the screws holding the three sections together.

All you have left now is to install the brass fittings in their proper places at the proper angles. For the last part, put on the filter, bale and glass bowl. After you do a few fuel pumps, you can take a few shortcuts. But if you use the method described above, you should gain the confidence to tackle rebuilding other components using this same procedure of disassembly, marking and reassembly. If you have any questions as to why the glass bowl came off first and went on last, contact your friendly rocket scientist or start hunting for a new glass bowl to replace your broken one.

One last tip: Before mounting the fuel pump onto the engine, turn the engine over until the cam lobe that operates the fuel pump is at its lowest point. The bolts go in a lot easier without having to fight the three springs in the pump.

See ya next month.

—Walt