VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc 1939 Fuel Pump

Today on the way home, driving my 1939 Master Deluxe 2 door, driving 45 Mph, all was going fine, then the engine started to buck as if it was running out of fuel. I slowed to 40 Mph and it smoothed out. Then later it happened again at 40 Mph, again I slowed to 35 Mph and it smoothed out. Happened once more and smoothed out at a slower speed. I suspect fuel delivery of my fuel pump. Does anyone know what the fuel pressure, vacuum side vacuum and fuel delivery of this pump is? I appreciate the help, Mike

One quick check is to disconnect the fuel line at the carburetor and crank the engine while you watch the fuel discharge. I cannot find a spec for the delivery rate.

The pressure should be in the 3 - 4 psi range. Much more and it will flood the carburetor. The tricky part is that you can have pressure but no flow.

I would also check for a restriction in the line going to the pump.

Well.... I thought I posted a reply, but for some reason it didn't take. I set up to measure the pump pressure and found that it was 5 pounds. My 1942 Motors Manual say 3 pounds is max. My son the Buick mechanic came over and helped out. I have a seciement bulb at the carburetor and we could see a few rust particles in it, but not much. We then did a flow test. I ran the engine until the carb went dry and I only got about 1/3 cup of fuel. I'm pretty sure that is not enough. So we removed the fuel pump and found the filter on top of the pump had a LOT of rust particles in it. Maybe 1/2 teaspoon. I have a vacuum bottle and pulled on the gas tank from the input of the fuel pump and the first pint of fuel was dirty. The second pint was nice and clean. So tomorrow I want to try and clean the pump and maybe it is OK, other wise I think I'll get a new one. Let you know. Thanks, Mike

Good troubleshooting technique. Your results are not unusual with these classic cars. I installed a Fram G2 filter at the tank outlet on my ‘37. It stopped the crud coming from the tank.

I did not have a filter between the pump and the carburetor. It was an old style pump with the glass bowl and screen.

While I have never tested it there have been other posts that recommend you not use multiple filters due to the increased restriction.

In the past I never had any high speed trouble, even with both so called filters. Actually, the fuel pump is an AC mechanical type AF pump. It has the glass bowl on top of the pump with a rock type filter element. The sediment bulb between the pump and the carburetor is a Carter sediment bulb from my old 1958 Chrysler Imperial and has the same type rock element in it. Later today I want to try and clean out the pump. Thanks for the help, Mike

This morning, and I mean all morning, I reinstalled the fuel pump. I had forgotten how difficult it was to start and tighten those TWO bolts. Anyway it's back in the car. I want to wait until my son can help me with the fuel delivery test, but I think I have another problem. I can see bubbles in the input of the fuel pump. You can see them thru the glass filter bulb. The inlet goes to a little stack and I bet that 1/2 of the inlet flow is air. After looking at the pump the only place where air can be pulled into the line is at the inlet pump connection and the gas tank connection (or the line has a pin hole, but I could not see and evidence of that). I believe that both connections are sealed, so that leaves the tank. I'm wondering if the pick up pipe may have rusted to the point that there is a pin hole that is in the air of the tank. Currently, I have less than 1/2 tank of gas. Suppose I can fill the tank up and see if the bubbles go away. Geezzzz...... it's always something, Mike

When I was at the point to make a decision to keep or replace my tank I bought a cheep camera to look inside the tank. Worked great cost about $30 on Amazon. Just put it down the filler neck and took a good look at everything. 5 minutes of looking around in there made my decision to replace the tank. Since that time I have used it to look into walls, cylinders and many tight places that would have required disassembly to get at before. Nice tool to have in the box.

Do you have steel fuel lines? If so it could be rusting from the inside out. I had that happen on an 04 Chevy truck.

I run a filter prior to the fuel pump. It is a glass inline one and while I have seen a few bits of stuff in it nothing major on mine. Just installed a inline one that looks like what should have been there prior to the carb. Have not had any issues running two on my car. Then again I have only had two on it for a couple weeks.

Say that's a good idea! I'll see what I can find. I filled the tank and the bubbles did not go away. Next I dripped oil on both pipe connectors. The connector at the suction side of the pump was the bubble problem. But that didn't fix the low fuel flow. I found some information stating that 25 seconds of engine cranking should deliver about a pint of fuel (16oz), my pump only delivers about 2 oz. So I called Chevy's of the 40's for a new pump.They do not have any and do not expect to get any until after September. I called the Filling Station and they also did not have any. A few more calls and still no soap. I did find a rebuild kit. Hopefully that will solve this problem. Let you know how the rebuild goes, once I get it and what pressure and fuel flow I get after the rebuild. Thanks, Mike

Well, shock and surprise, the rebuild kit came in the morning mail. So I changed out the parts. The old inlet check valve was definitely leaking. Even with my OLD eyes I could see that the rubber had failed. One item, the new pivot pin for the lever arm was sloppy and I could push it out with my finger. I measured it and it was 0.004" smaller than the old pin. I used the old pin. Re mounted the pump on the engine and tried the fuel flow test. I got maybe 6 oz this time, where I had 3 0z last time. I think that it should be more. I'm wondering about the eccentric on the camshaft. Maybe the pump lever arm is not moving far enough. Does anyone know what the travel should be on the pump arm. Maybe tomorrow I can remove the pump 'again' and set up a dial gauge to measure the eccentric movement, Mike

Did you hold the arm to pull the diaphram against the spring as you tightened the screws down, if this isnt done the diaphram has limited travel therefore limited fuel delivery.
Tony