Fuel pump fuel flow rate

[Mike_Zahorik]

Recently my 1939 Chevy Master Deluxe JA developed a fuel pump problem. At speed the car would lop as if it were starving for fuel. Then going slower the engine would smooth out. My general question is, what is a good fuel flow rate for a fuel pump of this vintage. My fuel pump is a AC mechanical pump model AF. I set up a flow test and found that the old pump would deliver about 3oz of fuel in 30 seconds. After rebuilding the pump I could get 6oz in 20 seconds, which is about 3 times as much as the poor pump. I read somewhere that a mech fuel pump should deliver about a pint (16oz) in 30 seconds, but maybe that is for more modern pumps? I have cleaned the tank, lines sediment bulb and carburetor. I also measured the travel on the eccentric of the camshaft and found it to be nearly 0.300 inch (I believe this is supposed to be 0.250"). I think I'm at the point of just re assembling this mess and just seeing if the problem has been corrected. Should would be nice if there were some tech spec's on some of this old stuff, or have I been looking in the wrong places, Thanks for the help, Mike

[TerryB]

Using a vacuum gauge you can test fuel pressure to the carb too if you wish. Typical is 4-6 psi at fast idle.  Flow rates are sometimes listed in the shop manual for your car.  

[Bloo]

1 hour ago, Mike_Zahorik said:

Should would be nice if there were some tech spec's on some of this old stuff, or have I been looking in the wrong places, Thanks for the help, Mike

Well if it's not in the shop manual(?), and I since don't have one to check, it's time for a SWAG....

 

So, lets say you are going 60MPH (that's really pushing it in a 39 Chevy). Lets say it gets 10MPG at speed. It's probably not that bad. If you were already moving, and the carburetor was already full, how much fuel would it take to maintain that speed?

 

60mph  60 mlles = 1 hour

 

10mpg   60 miles=6 gallons

 

6 gallons 1 hour = 6gph / 60 min = 0.1GPM

 

Ok so 0.1GPM to keep the car moving at a 60MPH/10MPG steady state. Since your measurements are in oz. (I assume fl oz.)

 

1 gallon = 128 fl oz.

 

0.1 gallon = 12.8 fl oz.

 

0.1GPM = 12.8 fl oz. per minute

 

You had:

 

3 fl oz. per 30 sec = 6 fl oz. per minute

 

And now you have:

 

6 fl oz. per 20sec  = 18 fl oz. per minute

 

So you have tripled the output of the pump. It definitely needed the rebuild. Also 6 fl oz. is not enough, but that is probably a little misleading. I don't know what RPM you made the test at (I assume idle?), so the output would have been a lot better at speed. In real life driving you would need more than my steady-state 12.7 fl oz. guess, but you have 18 fl oz., and that is going to get higher at speed too.

 

1 hour ago, Mike_Zahorik said:

I also measured the travel on the eccentric of the camshaft and found it to be nearly 0.300 inch (I believe this is supposed to be 0.250").

 

Good that you checked that. Most people don't, and they completely miss problems there.

 

1 hour ago, Mike_Zahorik said:

I think I'm at the point of just re assembling this mess and just seeing if the problem has been corrected.

I would, and if it isn't, I would start looking at the ignition. Good luck.

 

Edited May 30, 2023 by Bloo (see edit history)

[Mike_Zahorik]

About the only information I found in a manual was that the pressure should be 3-4 pounds. Actually, both the old and the rebuilt pumps had around 4#.  I seem to remember that some of these old carburetor float needle valves can not hold back 5#. The flow test was at cranking speed, less than idle, so you are probably correct in that 6 oz is low. Suppose I could rig up a temp fuel supply to the carb and measure again. Maybe if the problem comes back.

 

I really like your SWAG. You are correct in thinking that 60 MPH is pushing it. The engine is screaming around 50 MPH. My gas mileage has been between 14 and 15 MPG. 12.8 oz/minute does seem a little low, especially if that one pint in 30 seconds is correct, yet I have put the engine back together and it seems to run just fine. Time will tell. AND, after closely inspecting the inlet check valve, it was definitely leaking. Thanks for the help, MIke

[pmhowe]

I really like this kind of thread: Problem identified, solutions explored, good advice given, problem solved - most likely - we will see.  Thanks, Mike for posing the problem. Thanks, Bloo and Terry B for good advice given. We need more of these kinds of threads.

 

I just faced a similar problem with my log splitter. It has been running somewhat irregularly for some time. Last week I was trying to split a cord or two of wood and the engine went from running, but searching, to searching hard and then failing. If I waited a few minutes, I could restart and run for about another five minutes, at which point it stopped.

 

This suggested the carb was not filling fast enough to keep the engine running. I disconnected the fuel line at the carb and, to my surprise, no gas came out. I was fairly sure the tank was full, but took the cap off to check just in case my memory had failed again. (I'm not getting old, but I'm superannuated.). To my surprise, once the cap was removed, gas started pouring out the line to the carb. It turns out the cap has very small vent holes that apparently had gotten mostly plugged over time. I blew the air holes out with my compressor. Splitter now runs better than I do.

Phil

[32buick67]

Hopefully this helps, looks like one pint in one minute for model AF:

 

[32buick67]

[Stude Light]

You can read through this post from my experience on my 1939 LaSalle and maybe it’ll give some ideas (ignore the discussion on engine knock -resolved that) 3-1/2 psi is the number for my car. Once it starts getting much above 4 psi it starts pushing past the needle. 

 

Edited June 1, 2023 by Stude Light (see edit history)

[TerryB]

On 5/30/2023 at 9:21 PM, pmhowe said:

I really like this kind of thread: Problem identified, solutions explored, good advice given, problem solved - most likely - we will see.  Thanks, Mike for posing the problem. Thanks, Bloo and Terry B for good advice given. We need more of these kinds of threads.

 

I just faced a similar problem with my log splitter. It has been running somewhat irregularly for some time. Last week I was trying to split a cord or two of wood and the engine went from running, but searching, to searching hard and then failing. If I waited a few minutes, I could restart and run for about another five minutes, at which point it stopped.

 

This suggested the carb was not filling fast enough to keep the engine running. I disconnected the fuel line at the carb and, to my surprise, no gas came out. I was fairly sure the tank was full, but took the cap off to check just in case my memory had failed again. (I'm not getting old, but I'm superannuated.). To my surprise, once the cap was removed, gas started pouring out the line to the carb. It turns out the cap has very small vent holes that apparently had gotten mostly plugged over time. I blew the air holes out with my compressor. Splitter now runs better than I do.

Phil

This is the stuff that keeps me going.  My physical condition limits what I can do to just basic stuff so these challenges are just what the doctor ordered to keep my mind (somewhat) sharp😀.  The guys that have done lots of this stuff really come through with good suggestions and that has helped me learn a lot more thanks to them.

[Mike_Zahorik]

I'm glad this help others, besides myself. I have driven the car quite a bit since the rebuilt pump and it runs fine. It is interesting that the 1 pint per minute works out to 7.5 gallons per hour, which is less that the numbers above. So I deem this problem solved, Thanks for the help, Mike