fuel delivery issues - next steps

[tcslr]

working on sorting the fuel system in a 1931 Buick.  the car starts well, idles well, but after say 10-15 minutes and under load, acts as if it is lean, pulling choke out fully does seem to help - but fuel starved

 

This is a dual point system.  accurately timed. but modern fuel ( thank you Mr Obvious) - it is timed as Buick recommended. 

Vacuum tested the fuel line to pump - ( with a MityVac tool) - pulls fuel well.  The buick sending unit has a vent.

tested pump discharge - it would go from 1lb then decay - pretty fast to 0.

removed the model b pump, dis-assembled - no issues.

rebuilt Marvel carb.  My home is about 2K elevation and manifold vac is about 17ish. cracking ( quickly) throttle, manifold seems to properly respond wrt vacuum.

 

What are some additional testing I can do?  I haven't put pump back on and tested - maybe that is next step?  Any and all suggestions are very welcome.

 

 

[intimeold]

At our shop we would definitely start at the tank a proceed  through ; maybe the original 1931 fuel lines. 

 

Maybe you have already done that; but haven't said yet.  We have found stuff floating in vintage fuel tanks and causing problems. I would look there first; and proceed with caution.

[Akstraw]

Easier said than done sometimes, but can you rig a temporary fuel supply from a clean tank?   If problem disappears, then you know it is in the tank/lines.  I have a plastic tank from an old lawn mower that I keep handy for just such evaluations.

[Stude Light]

Had the same issue in my 1939 LaSalle. It was the fuel pump. To diagnose it I put a tee in the line from the pump to the carb and hooked that up to a fuel pressure gauge I could read while driving.

My mechanical pump ran at 2.5 psi while idling.

I started with the engine warm enough to open the choke and be driven normally and found when I accelerated hard up through the gears the mechanical pump would run down to 1.5 psi at the worst. Once the engine got up to operating temp and it was hot under hood, I could get the pressure to fall to zero. If I kept accelerating hard the engine would stumble.  I also flow checked the pump cold vs hot and found it would barely pump when hot. I took the pump apart and everything was fine inside but what I finally discovered is that the check valves would start to leak past at higher temps so a rebuilt pump solved the issue.

Scott

[dodge28]

Sometimes we overlook the simple problem, It is just natural. I was a new employee at a shop when the manager asked me to fix the shop forklift. Problem was frequent stalling. After about 10 different mechanics failed to correct it it was handed over to me. I found a sales receipt floating at the bottom of the fuel tank. In the early eighties the craze was fiber glassing fuel tanks inside by putting fiber glass mixture and swishing it around. A friend had this done to his Buick tank The car was stalling all over the place .A few tow job . I checked the car over with a fine tooth comb until I found a piece of loose fiber glass was moving around in the tank. The solution was a new tank. I am not saying this is your problem. But do not over look crud in the tank. I think Akstraw has a good idea,

Cheers and good luck

[Dave39MD]

I found a lot of threads about this when tracking down the same problem in my 31 Buick. I repaired a lot of small problems which when corrected improved performance. Heat riser tubes had small holes, heat riser blocked off at carb instead of valve, intake gasket improperly installed causing vacuum leak on one runner, fuel line from pump to carb getting very hot so it was wrapped with insulation, fuel pump rebuilt, and finally the carb gone through a couple of times.  Many corrections were made at the same time so I don't know what the final fix was but it is running great now.

 

One more was using non ethanol fuel.

 

Dave

Edited May 5, 2023 by Dave39MD (see edit history)

[DFeeney]

My 1931 Chrysler had a dual point distributor.  I thought they were over complex.  4 plugs fired on one  set and 4 on the other.  I installed a single set distributor from a 1936 Chrysler and ran the car for years with no problems.  It was a bolt in.  Perhaps a later Buick distributor  can do the same for your engine.

Edited May 7, 2023 by DFeeney (see edit history)

[Mark Shaw]

Easy things first....  Verify the fuel tank cap vent is working. 

[tcslr]

The sending unit on the 31s have a vent. 

[Mark Shaw]

11 hours ago, tcslr said:

The sending unit on the 31s have a vent. 

OK, but is it working?  You can test it by simply removing the gas cap on the next run.

Edited May 7, 2023 by Mark Shaw (see edit history)

[Rusty_OToole]

On 5/5/2023 at 6:03 AM, Dave39MD said:

I found a lot of threads about this when tracking down the same problem in my 31 Buick. I repaired a lot of small problems which when corrected improved performance. Heat riser tubes had small holes, heat riser blocked off at carb instead of valve, intake gasket improperly installed causing vacuum leak on one runner, fuel line from pump to carb getting very hot so it was wrapped with insulation, fuel pump rebuilt, and finally the carb gone through a couple of times.  Many corrections were made at the same time so I don't know what the final fix was but it is running great now.

 

One more was using non ethanol fuel.

 

Dave

Reminds me of a VW camper van of about 1980 that I worked on years ago. It had about 100 vacuum hoses on the engine and it seemed they were all cracked and leaking. Every time I fixed one it ran a little better. After a couple of days work it ran great, when I started it would barely run. None of these things cost anything to fix it was a matter of diagnosing finding and fixing them all. Have had similar jobs where 2, 3 or 4 small faults added up to a big problem. The oldest mechanic in the world could not diagnose this kind of thing from a distance.

[MikeSampson]

Given you've rebuilt the Marvel carb and tested various elements like pump discharge and vacuum, I think reinstalling the pump and testing it would indeed be a sensible next step. If you haven't done so already, consider pressure testing the entire fuel system as one unit. Sometimes, issues can arise from how components interact rather than how they perform in isolation.
For some more targeted guidance and possible solutions, you might find https://stuck.solutions/ helpful. They've got a range of articles covering all sorts of mechanical woes, including fuel delivery issues.

Edited September 17, 2023 by MikeSampson (see edit history)

[drhach]

I love that external fuel pressure gauge. I had a similar issue and installed a gauge as well. Although, I wasn't clever enough to make it readable from the cockpit. Rebuilding my fuel pump solved my problems. Measuring is the best way to know for sure. 

 

 

[DFeeney]

As mentioned I would clean/ replace the gas tank plus all the fuel lines.  If they slightly. occasionally  leak due to age the fuel pump would suck air not gas.  Keep us posted as to your fix.

[demco32]

Let the engine run on a separate tank and fuel pump.