The Stovebolt Site Forums The Shop Area Engine Shop Fuel pump issue?

My truck has a GMC 270 cubic inch motor. When I accelerate above a normal amount it seems to cough and stumble. I let off the pedal and it catches up. When going up hill and trying to keep up speed it will also cough, doing the same thing. I let off the pedal a little and it runs ok.
I feel the fuel pump is not supplying sufficent fuel to the motor.

What say ye motor Gurus?

Be sure you have a new, clean fuel filter. Even the best pump can't push enough gas through a dirty filter to meet the demands of open throttle running. What were the results of the fuel pump volume and pressure test- - - -the one I'm sure you must have done by now?
Jerry

Unfortunately I don't have those tools in my box. Please enlighten me.

A 1 quart oil bottle and a vacuum/pressure gauge- - - -total investment less than 20 bucks. Every shop needs that gauge. Like the slogan Sun Electric Co. used to put on all their equipment- - - -"We test, not guess!"

www.ebay.com/itm/404764845848?

The pump should produce at least a pint of volume in 30 seconds at idle speed, make 2-4 PSI of pressure and hold it for a short time after engine shutdown, and hold at least 12 inches of vacuum on the suction side for a short time after engine stop. If it passes the pressure and vacuum test but is short on volume, fix the restricted fuel line between the gas tank and the pump. A collapsed flex line between the frame and the engine is a good place to start looking.
Jerry

How do you measure the fuel output with the engine idling? Do you run the engine with an electric pump with the mechanical one disconnected?

A manual I have for another car says to measure the pump volume of 9 squirts of the fuel pumpinto a jar while cranking. It should total 8oz. Is there a similar test on the pump that can be done with an AD truck?

If the engine won't idle for 30 seconds on the fuel in the float bowl, fix the carburetor, or whatever else is wrong. Just disconnect the fuel line and put it into the catch container. For the pressure and vacuum test, just refill the float bowl through the vent tube with a turkey baster.
Jerry

Next time you drive your truck, pull the choke knob out to partially close the choke plate to see if the stumble goes away.
If it goes away, you likely have a carburetor issue. Likely dirty requiring a rebuild.
Partially closing the choke increases the vacuum which forces more fuel to enter the venturi through a partially clogged passageway.

Thanks

I knew this would be the place to get answers.

Now I have to buy a new tool!

Gauge and filters on order. I'll check back in a few days.

Update:

I changed the filters, did a test drive, then I did some tests with the pressure gauge.

On the pressure side it's making 5.4 and holding at idle. After turning off the motor, it held at 5.1 for over 5 minutes.

On the vacuum side it was pulling 9.9 at idle.

During the driving it didn't stumble until I got it above 60 mph. That's Ok since I don't drive near that speed.

Are these numbers acceptable?

5+ PSI is a bit too high- - - -right on the ragged edge of overcoming the inlet needle valve in the carburetor and causing flooding and stalling at hot idle. 4 or less would be better. How is the volume? With that kind of pressure and an unobstructed line from the tank to the pump, it's probably OK. What carburetor are you running?

I have a Weber 32/36 on it. I was having issues with the Zenith previously. It was constantly weeping gas on top of the manifold.

I didn't check volume.

9.9 inches of (manifold, not ported) vacuum is way too low. Should be almost double that and steady.

The Weber makes a pretty good sinker when bottom fishing for big catfish. It is also designed for less than 3 pounds of fuel pressure. A Zenith is the only carburetor better than a Carter. Fix it and put it back on. If you insist on running the Weber, get a GOOD fuel pressure regulator.

Bill- - - -the suction side of the fuel pump is holding 10 inches of vacuum even after engine shutdown- - - -that's good, and it's a test VERY few people have been willing to do over the past 20+ years when I suggested it.
Jerry

Jerry, I feel like my Zenith is pourous. I had National Carb do a rebuild on it and it still seeped.

The vacuum test was fairly easy, even with my fat stiff fingers in a tight spot. I had to hang over the headlight and twist the fitting with my left hand because my right hand wouldn't turn around backwards.

Do you have any recommendations for who to have fix my carb?

General Motors used to recommend a specific sealer for leaky carb castings back in the 1980s- - - -something that usually happened after a rebuild when certain dip cleaning chemicals were used. Apparently, they had a run of bad castings that needed to be sealed to prevent leaks, and the dip cleaner would dissolve it. I don't have any idea if it's still available- - - -probably not. Some of the stuff we used back then can have a bad side effect on the environment- - - -they'll make your kids be born naked! Jon (Carb King) might know something about the casting sealing process.
Jerry

You might try painting the outside of the casting with polyurethane clearcoat with an isocyanate hardener added. (Similar to the old DuPont IMRON) Here's an example of the hardener I'm thinking of.

www.ebay.com/itm/374191568234?

Caution- - - -isocyanate hardener can be toxic- - - -cover all exposed skin and use a respirator while spraying it.
Jerry

Isocyan(ide)ate.

Sorry, I thought he was reporting manifold vacuum.

Question: How do you do this test? Because the other end of the line attached to the "vacuum" side of the pump terminates in the gas tank, you'd have to block it off in order to develop a vacuum. And when you did that, you'd just have the gas in the carb to run on while the vacuum built up.

Just trying to learn.

Bill, I disconnected the fuel Line from the inlet side of the pump. Luckily the pump is higher than my tank so I didn't have any leaking.
Connected the gauge to the pump, started the motor, let it idle and got the reading.

All these tests, volume, pressure, and vacuum, depend on the engine being able to idle on the fuel in the float bowl for a short time- - - - -30 seconds for the volume check, less than half that time for pressure and vacuum checks. Any engine that won't do that has far worse problems than a flaky fuel pump. I've been doing that test since the early 1960s, so I'm not just parroting some pie in the sky theory from somebody's website. I try to share real world facts and figures that have served me well for decades. Maybe that comes from being the middleman in 5 generations of mechanics- - - -from my grandfather in the 1920s to my grandson, who is currently making a living fixing cars. His 3 year old daughter had her tricycle turned upside down on the floor a few days ago, "working on it!"
Jerry

This test is what I did to check the fuel output of the mechanical pump on my trucks.