Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

[Russ T.]

I have gas spurting out of the primary vent tube of a newly rebuilt Holley #2818 carb. The car starts and idles well, it's just
gushing gas while running. And it's quite a bit of gas. The car has about 75 miles on the rebuilt carb and engine. The fuel tank is original with fresh gas. I've driven this '64 convertible since 2004 and never had such an issue. I thought that I could have something clogging the needle and seat assembly. I tried the following procedure to investigate:

• Tapped on the carb with a wrench to try to dislodge any blockage. No success.
• Disconnected the fuel inlet line and let all of the gas run out of the carb before reassembling and starting again

• Removed the primary fuel bowl to check out the float and spring. Looks fine, no cracks/holes/gas in float.
• Changed out the needle and seat assembly to a Holley P/N 6-504, a Viton needle and seat assembly.

I just can't make it stop gushing gas. My next stop will be to tee-in a fuel pressure gauge either right out of the fuel pump
or between the fuel filter and carb.

After that I could install the carb on one of my other cars and see what happens.

I know that the rubber diaphragm can fail and results in a loss in pressure.
Is it possible for the fuel pump to fail and pump excessive fuel?

Thanks,
Russ Trotter
NCRS #15044

[Frank D.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

Why wouldn't you conduct a simple 10 minute test with a $20 vacuum/fuel pressure gauge and check the pressure and volume of the fuel pump? See attachment. It is my understanding that Holley carbs like around 5-1/2 lbs of pressure. Once exonerated, you can concentrate on the carb as the source of the issue.

[John D.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

Russ, Adding to what Frank said here is what I go thru with fuel injection restorations. Owner using a engine fuel pump putting out two much pressure. John

[Harry S.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

The link below tests for low pressure. Use the same setup to test for too much pressure.

https://www.forums.ncrs.org/showthread.php?124646-63-4657-Fuel-Pump-Low-Pressure&highlight=pressure

[David M.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

If its not over pressure from the pump, which is rare, then its a likely a float or needle & seat problem. Either could be hanging up. All it takes is a small piece of debris between the needle and seat to keep the float from sealing off. Or if its a plastic float it may be absorbing fuel causing it to... well... not float.
Check the fuel filter for contaminates(cut open). If its blocked it bypassing sending debris directly into the bowl.

[Bob R.]

[Joseph S.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

First, of course you should verify float, needle and seat are all clean, float doesn't have any small cracks (carb rebuilders will tell you from excessive heat) and are adjusted properly.
However I have seen overheated fuel (boiling) create dripping from jets and accelerator pump nozzles. Standard pump 91-93 octane fuel with 10% alcohol has a lower boiling point than the fuel these engines were designed to run with. Especially if the fuel you are using was a winter rather than summer blend. Aviation or Racing fuel may overcome this issue or in addition to other mods to insulate the carb from heat soak. (insure your heat riser valve isn't stuck closed)
Just something to consider nothing else works.

[Russ T.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

Bob,

This is where I will start investigating this weekend. A new replacement pump was installed
rather than an old original one, so I will be looking closely at that first.

Thanks,
Russ

[Russ T.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

Resolution:

After getting satisfactory readings on a fuel pressure test and a fuel volume test, I removed the primary float bowl once again
to check out the float. This time the float was completely full of gas, and staying submerged instead of sealing off the gas flow.

There were no pinholes in the float, so I guess gas seeped in through the seam in it.
You could shake the bowl and hear gas sloshing around inside. Apparently I failed to notice this the first time
I checked it. There was much less gas inside, but probably enough to keep the float weighted down.

I bought a new float from Summit ($14.00) and everything was back in order once I got the float adjusted.

In retrospect, a problem like this can usually be caused by 1 of 2 things only:
1) Gas is leaking by the needle & seat due to a poor seal from impurities, etc.
2) The float is not closing the needle due to restrictions in the movement of the float. In this case,
extra weight from absorbing gas.

Over-pressurization from the fuel pump seems highly unlikely, but not impossible.
I read a link on the AACA web page that stated sometimes an improper rebuild of the pump
can cause such characteristics if the diaphragm is stretched too tight, resulting in higher pressure.
An output of 7 psig is enough to cause flooding problems.

However, I'm going to classify that as a "long shot." From now on, my focus will be on the float and
the needle and seat assembly in such a situation.

Special thanks to all of you guys for your valuable input.
I'll try to "pay it forward" in the future!

Russ Trotter
NCRS #15044

[Timothy B.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

Glad you posted, Russ, and glad you found the problem.

[Paul D.]

Re: Can a Fuel Pump Failure Result in Excessive Fuel Flow or Pressure?

Russ, be sure to check your oil level for overfill and give the oil the "sniff test" for fuel contamination. Any indication of fuel in crankcase would warrant an oil change.