Series 40 AC Fuel Pump Pressure problem

[eric_b_1937]

I have rebuilt two AC fuel pumps for my 1937 series 40. Both pumps have had a similar problem. At highway speeds the pump over time cannot keep the fuel supply at a rate required. Then I will have a small backfire. I have a clear fuel filter at the carburator. At Idle the pump does not shoot fuel into the filter with any pressure it just pours in. I checked for clogging and the lines are ok.

Does the spring that comes in the overhaul kits have a different spring rate then the originals thus Giving a different pressure output? I have been using the original springs in my pumps. I rebuilt a third pump last night and I used the original springs in it should I replace the springs before testing? If this third pump will not keep up the fuel supply I'll buy a electric pump.

[Buicksplus]

I think your car is suffering from fuel vaporization, a common problem with old cars and modern gasolines. No need to keep fooling with the mechanical pump, it is probably working as designed and it is unlikely the springs or other internal parts are at fault. You might make sure the arm is making proper contact with the cam and is receiving the full stroke as the camshaft turns.

I'll bet this problem occurs when the engine is hot. Does it happen right after a hot start? If so, it may clear up if you keep the car moving. This is because incoming fuel finally cools the carb body enough so that the boiling of fuel in the carb stops.

If it continues, check out the fuel line routing to make sure it says away from hot exhaust manifolds or other sources of heat.

Make sure the carb lower gasket is thick, it is an insulator designed to keep the carb from getting too hot from the manifold exhaust heat. If your car has an exhaust damper valve (I am not sure the 37 had one), make sure it is not stuck in the closed position -- this will add much more exhaust heat to the intake manifold and carburetor.

An electric pump can help, but they are not a cure-all. If the fuel is boiling bad enough, even an electric pump cannot overcome the problem.

Don't use ethanol laced gasoline, it has a much lower boiling point and this makes the problem worse.

Make sure the cooling system is working OK, any old engine if hot enough will succumb to vapor locking.

That's my 2 cents, you'll probably get quite a few more suggestions.

Good luck!

Bill.

[eric_b_1937]

Thanks for your input Bill.

I had a machinist rebuild engine eight months ago. I have 700 miles on it so it?s still pretty tight. I?m test driving and working the bugs out of it now. When I started the car in the morning it started perfectly, drove it 10 miles to work. Drove home 10 miles then had problems starting after sitting for a few minutes so it could be hot gasoline. Hot gas equals high line pressure at the carburetor so the outlet valve on the fuel pump does not open all the way?

I?m using regular gasoline. The fuel line is routed at least 6? from the engine and manifolds. Insulator is about 1.5? thick. Engine temp was 180 deg. The exhaust damper valve is free and has a new thermo coil on it. I will fix exhaust damper in the open position and see the results.

I was thinking about the cam being shaped wrong too. I didn?t personally grind it so I have no idea if it?s making full contact and is receiving the full stroke as the camshaft turns.

Who out there tours their straight 8 cars long distances? I?m talking well over a thousand miles per year.

[Buicksplus]

Eric:

We had a '39 90 series with a 320 straight eight. I drove it on thousands of miles of tours, including one round trip from Albuquerque to Flint, MI in the very hot summer of 1988. She had vaporization problems whenever started hot, mostly bucking and gasping for the first few miles. Then it would clear up once the fuel cooled the carb off. What happens is the fuel boils in the bowl and all the lines in the engine compartment which leans out the mixture too much to support smooth running.

It ran fairly hot and boiled over often at first but once I cleaned the radiator and rigged up a 6 bladed RV fan on the engine, we could drive it in 100 degree weather -- though always with one eye on the temp gauge. The more we used the car, the better it cooled, I think it was stiff from the rebuild when we started driving it.

These prewar engines are very capable, though we generally drove ours around 60 MPH or less. They will go much faster of course, and they are smooth and quiet -- but the piston speed and bearing loads go up sharply with RPM. The fact is there were not that many roads pre-war where you could drive all day long at 75 MPH like we have today.

Keep driving the car and you will gain confidence and experience with its quirks.

If you add an electric pump, I would avoid the cheaper 6V "Autopulse" pump that is available from many sources like JC Whitney or AutoZone for $35 or so. NAPA sells a 6V Carter rotary pump that is much more capable and costly. If you use the Carter, consider by-passing the mechanical pump completely. Putting the two pumps in series creates problems overpressuring the carb when the electric is on, or putting extra resistance in the fuel line when the electric is off. The Carter rotary is big enough to supply all the fuel your engine needs. But you will still have problems with hot starts, that is just due to the physics of boiling fuel in the carburetor.

I doubt if the that pump cam was ground off, surely the machinist knew not to do that. I think if you take off one of the diaphram covers you can check the stroke of the pump buy turning the engine one rev by hand. This is not likely to be a problem, but I would check it anyway just to make sure.

Good luck and keep on drivin' them....

Bill.

[Mark Shaw]

I suggest you disconnect the pump from the carb. and crank the engine to see how much gas you can pump into a can. If it is pumping gas, your problem is heat.

The best solution for a hot gas problem is a bypass line back to the tank to keep cool gas going to the carb. Put a tee behind the carb. & connect the fuel line to the end of the tee. Put a pressure regulator on the side outlet of the tee and run a return line back to the tank. This will keep cool gas running to the carb.

[carbking]

Two other issues to check:

Fuel filter and tank vent.

Jon.

[eric_b_1937]

I found some info in a buick maintenance tips book that said the pump should put out 2.5 - 3.5 psi, I might try to hook up a pressure guage to the line. I believe the tank is vented fine, the filters are clean. I have not visually checked the inlet inside the tank yet, but I back blew it with air. Great Idea for a T line, I've seen photo's of cars with that but I didn't know the purpose.

I put a pressure guage on the line and it reads around 1 psi at Idle and the same with higher RPM's. I'm going to swap the pump with another one I rebuilt. I still do not know why the pump on the car is not putting out the right pressure. I'll tear the pump down and look for any problems again, but probably won't find anything.

Does anyone have AC fuel pump data that can support 2.5 - 3.5 psi output?