[Stan R]

My '71 GTO has the 400 engine and Quadrajet 4MV carburetor (29865). I recently replaced the AC fuel pump with an aftermarket pump (Precision brand) because the 41382 AC Delco is no longer available.

When I started the car after the new pump was installed, gas flowed out of the accelerator pump at the top of the carburetor. I then removed the new pump and installed the old AC pump and the problem disappeared. I drove the car and still found no leaks afterwards. I also subsequently installed a second new pump (Carter brand) and experienced the same leak problem, but based on the appearance of the two new pumps and the "Made in Korea" label on the boxes, I suspect that they could have been identical products made in the same factory.

Any ideas on what's going on? Is it possible that the new pumps exceed the maximum pressure spec, or is the issue in the Quadrajet? Any advice that can be provided would be greatly appreciated!

[steve25]

My moneys on the two over sea’s made new pumps outputting far too much pressure!
You could add a regulator if your not concerned about looks too much!

[DavesGTO]

My Dad had the carb rebuilt for his 65 and put a new cheapo fuel pump on at the same time. Car wouldn’t start so he took the carb off and went back to the place that rebuilt it, guy said the Chinese fuel pump he put on was putting out too much pressure. I put a regulator on the fuel line and it started right up, still there to this day. Dad was tight.


Sent from my iPhone using Tapatalk

[Tom Vaught]

I would agree with steve25 on his diagnosis.
Most likely the same two pumps, the wrong spring for the application, but a basic fuel pressure regulator would work. The Holley 12-803 unit ($37.36 from Summit) is probably the cheapest out there and should lower the pressure to about 5 or 5.5 psi with an adjustment. Some plumbing required.

Tom V.

[mchell]

Originally Posted by Stan R

My '71 GTO has the 400 engine and Quadrajet 4MV carburetor (29865). I recently replaced the AC fuel pump with an aftermarket pump (Precision brand) because the 41382 AC Delco is no longer available.

When I started the car after the new pump was installed, gas flowed out of the accelerator pump at the top of the carburetor. I then removed the new pump and installed the old AC pump and the problem disappeared. I drove the car and still found no leaks afterwards. I also subsequently installed a second new pump (Carter brand) and experienced the same leak problem, but based on the appearance of the two new pumps and the "Made in Korea" label on the boxes, I suspect that they could have been identical products made in the same factory.

Any ideas on what's going on? Is it possible that the new pumps exceed the maximum pressure spec, or is the issue in the Quadrajet? Any advice that can be provided would be greatly appreciated!

Your qjet float could simply be a tad high and/or your needle and seat are suspect…….while your adjusting the float, put a new needle and seat in…..highly doubt the new (stock) pumps are putting out enough pressure to overcome a properly set float and a good needle and seat assembly

When it doubt put a pressure gauge on it …..

[77 TRASHCAN]

I think RobbMc makes 2 different Pontiac pumps, of course, they won't hook up to a factory fuel line.....

[Tom Vaught]

No one really knows what is happening with the carb/fuel pump UNTIL you put, as was suggested, a gage on the fuel line right before the carburetor.

If the fuel pressure is approximately 5-7 psi and the carb still has issues, time to look at what is going on inside the carb.

Tom V.

[Stan R]

Gentlemen, many thanks to all for the prompt responses. Probably won't be able to test the pump pressure at the carb without risking a bonfire under the hood, based on the rate that the gas is spilling out onto the engine. I think that the best approach is to assume that the pressure is above the 8 PSI max spec, and work on getting that corrected first. If I install the Holley regulator and adjust it to the minimum setting and I still have a gas leak, I'll have to dig into the carburetor. Thanks again for all of your input.

[mchell]

Fwiw
Don’t assume ….a properly set float height and functioning needle and seat can easily handle 8psi or more without spraying fuel out the top of the carb…. Those stock canister pumps you are using have no chance of producing that kind of pressure.

In the time it takes to install a regulator you could have checked pressure and or look into the carb internals several times…

Also, how will you set up a regulator without a pressure gauge?

Been there….good luck with your project

[Tom Vaught]

The Gage tells you where you are at currently.
The regulator is a "fix". Carbs are designed to run at a set fuel pressure usually
from 5 to 6 psi. That is the specs that Holley, Rochester, Carter, and others used.

Sure you can run 8 psi on your track car to fix a marginal fuel system.
That might work for Drag Racing or some street driving but all of the durability of the
fuel system components is based off of a agreed fuel pressure specification.

If the carb does not leak fuel but has a different drive mode vs a stock set-up it is because you changed the settings that the carb manufacturers wanted.

EVERY CARB REBUILD SHEET inside a rebuilt kit gives specific settings for each carb.

Why would the carb people go to all of that effort and documentation if the carb drive and performance results were the same in all cases with "Run it at 8 psi fuel pressure".

Again the carb engineers did stuff for a reason. Put a gage on the engine vs listen to a
another opinion.

Tom V.

[mchell]

Wo there…

I am simply suggesting that if the poster is using a STOCK mechanical pump AND he is getting fuel level rise in the float bowl that leaks out of the top of the carb he MAY want to check the carb internals first rather than last….. since two new STOCK fuel pumps exhibited the same behavior, maybe the old one has been putting out very low pressures while the new ones are bringing the pressures back to normal ranges.

Also, I am only illustrating that a carb COULD manage 8 psi plus without the leakage he is describing, PROVIDED the float, needle and seat are in good repair.

I AM NOT suggesting what pressure’s THEY SHOULD be running for their application.

[steve25]

The last time I had a AC Delco stock pump go bad on me I finally had a pressure gauge in the car.
It was outputting all of 3 psi of pressure and that was just enough to allow the car to make 28 to 30 mph on level ground and 10 mph up modest hills

[Tom Vaught]

To do the job right takes a given amount of time.
To do the inspection job several times (inspecting carb internals) says you don't have a clue what you are doing with a carb.
ONE INSPECTION of the carb internals should be enough.

Everyone has opinions, the owner gets the final choice in the matter.
If the owner wants to install a Holley regulator designed for 3 to 7 psi normal adjustment, using HIS MONEY, that is his decision.
It is HIS true street car, He gets to make the final decision. 47,000 posts stating 8 psi is ok for his application is 46,999 too many.

Good luck with your carb diagnosis. Have a good day.

Tom V.

[nUcLeArEnVoY]

A lot of these aftermarket fuel pumps make way too much fuel pressure. It's as though a lot of them are designed with springs and diaphragms meant more for late 70's applications when the Quadrajets had smaller floats, ergo bigger fuel bowls and were thus able to handle the extra pressure. Late 70's fuel pumps (such as the one that came on my '79 400) put out a pretty big amount of pressure (8.5-9.5 PSI), which is what you'll see a lot of these repro fuel pumps making... sometimes even more. I got tired of fighting fuel pressure issues, so I spent the $$$ and put in a correct NOS AC fuel pump on my W72 T/A.

Early 70's applications were more in the 5.0-6.0 PSI range.

Maybe try Carter, I usually hear good things about that brand? Or throw on a regulator.

[Kenth]

Fuel level in float bowl follows the fuel pressure.
Higher than standard pressure equals higher level, lower than standard pressure equals lower level.

1975-79 Pontiac Q-jets uses 17/32" float setting, 1971 units uses 9/32".
Using a 1977-79 7.0-8.5 psi Pontiac fuel fuel pump on an early Pontiac Quadrajet, that uses 5.0-6.5 from factory, requires lower float setting than standard.
Even at 7 psi you may experience some nozzle drip on 1974 and earlier units set at 9/32".
I have found a 13/32" setting will cure this with no ill effects.

FWIW

[Tom Vaught]

THANK YOU KENTH. I totally agree P@blo.

Tom V.

[Stan R]

Thanks for the additional input, Kenth. I also noticed that in a spec sheet that I saved from an old rebuild kit, the float setting for '73-'74 400's is 13/32"...exactly what you recommended.

Except for B-Series vehicles, the '71 Pontiac Service Manual calls out a fuel pressure range of 6.5 PSI to 8 PSI for all 4bbl V8 engines. I could try doing a pressure check on the new pump by attaching my Sun gage to the disconnected fuel line at the carb, assuming that I can get the car started and keep it running at 1,000 RPM on the gas in the bowl long enough to get a reading. If the pressure's within spec then it's time to open up the Q-jet and take a look at the float setting, or just do a total re-build.

[mchell]

Originally Posted by Tom Vaught

To do the job right takes a given amount of time.
To do the inspection job several times (inspecting carb internals) says you don't have a clue what you are doing with a carb.
ONE INSPECTION of the carb internals should be enough.

Everyone has opinions, the owner gets the final choice in the matter.
If the owner wants to install a Holley regulator designed for 3 to 7 psi normal adjustment, using HIS MONEY, that is his decision.
It is HIS true street car, He gets to make the final decision. 47,000 posts stating 8 psi is ok for his application is 46,999 too many.e internals

Good luck with your carb diagnosis. Have a good day.

Tom V.

If you are referring to my comment on inspecting the internals several times, you have missed my point entirely. Let me clarify if you didn’t read my initial comment correctly:

What I was i was suggesting was simply that you could inspect the internals of a carburetor several times in the time it would take to install a regulator. Meaning why not inspect the internals first?

I don’t give a lickty shi$& what the poster chooses to do with HIS car……I did supply an opinion to a problem which I believe is how these forums work.

I’ll ignore the comment about not having a clue, even tho I want to tell you to f$&& off….I’ll refrain

[Tom Vaught]

Tom V.

Could the Original Poster post up some history about the carb and the car.

[Sirrotica]

To start with, the car doesn't have to be running to get a fuel pressure reading. Disable the ignition, and dead head the gauge, turn the engine over with the starter to get the reading. As long as the engine is spinning at a couple hundred RPM you should be able to find out if the new fuel pump is faulty, and over pressurizes the carb. It's not the prescribed way to test it, but it should tell you if it's over spec. It also won't set the car afire.

One other problem I've seen pertaining to over pressurizing a carb on a Pontiac, is a fuel pump cam eccentric installed improperly by someone not familiar with proper installation on the front of the camshaft. It can cause the stroke of the fuel pump to be too long, and cause too much pressure. It's not common, but I've seen it twice in my career. both times it was either a timing chain, or camshaft replacement, done previous to the problem. The old pump could be bad enough that the problem goes away with a worn out pump.

One other thought is either crap in the new fuel pumps getting in the needle and seat, or a sliver of neoprene from the hoses from changing pumps getting into the needle and seat. Theoretically it shouldn't get past the OEM filter, if the filter is still in service, but I've seen all kinds of crap get into needle and seats with the OEM filter still in place. Lots of sediment gets past them as anyone that has taken a carb apart can attest to.

If the filter has been deleted then foreign matter is definitely a possibility.

Agree that until there is a gauge test done, everyone is shooting in the dark, and working on assumptions.