Fuel pump

[Huckleberry]

1978 GMC K15. Stock crate 350ci. Mechanical fuel pump(stock). Does a fuel pump go weak or just quit? Truck sometimes starts hard but not always. Truck runs good, then lays down when throttle applied. (45-50 mph). Sometimes stalls out. Truck will restart. Replaced the distributor and Carb 1 year ago. I not sure sure thanks in advance.

 

[75gmck25]

It could be a weak pump, but usually they just leak. However, I would also check the spring and Quadrajet fuel filter to make sure it is correctly installed. It’s an easy mistake to make. Also check the float level and the needle to make sure you aren’t just emptying the float bowl and running out of fuel.

Also look down the carb and verify you get a good accelerator pump shot when you move the linkage.

If you are willing to pull the top off the Quadrajet, make sure the rods are pulled down with vacuum applied, but they rise up as vacuum drops. The rods are pushed up by a spring at first, but then pulled back down by high idle vacuum to the cruise position (lean). However, the springs then move the rods back up to power mode (richer) when vacuum drops. It sounds like you have no power mode.

 

[Ricko1966]

Is fuel tank vented correctly

 

[fast 99]

It's possible to check for stuck metering rods by inserting a small screwdriver down into the bowl vent. When rod holder is lightly pushed down it should spring back. If not metering rods are stuck.

Also, fuel pumps today are suspect. Partially from ethanol and usual poor quality. Check fuel pressure and volume.

 

[Edelbrock]

1978 GMC K15. Stock crate 350ci. Mechanical fuel pump(stock). Does a fuel pump go weak or just quit? Truck sometimes starts hard but not always. Truck runs good, then lays down when throttle applied. (45-50 mph). Sometimes stalls out. Truck will restart. Replaced the distributor and Carb 1 year ago. I not sure sure thanks in advance.

When its hard to start, give it a shot of fuel or starting fluid. If that makes it start right up, then you are lacking fuel. If it makes it worse, you were already flooded.

 

[jfrchevyguy]

I just had the very same issue with my '76 C20, and yes I replaced the fuel pump and now it runs and performs as it should.

 

[Huckleberry]

It could be a weak pump, but usually they just leak. However, I would also check the spring and Quadrajet fuel filter to make sure it is correctly installed. It’s an easy mistake to make. Also check the float level and the needle to make sure you aren’t just emptying the float bowl and running out of fuel.

Also look down the carb and verify you get a good accelerator pump shot when you move the linkage.

If you are willing to pull the top off the Quadrajet, make sure the rods are pulled down with vacuum applied, but they rise up as vacuum drops. The rods are pushed up by a spring at first, but then pulled back down by high idle vacuum to the cruise position (lean). However, the springs then move the rods back up to power mode (richer) when vacuum drops. It sounds like you have no power mode.

Thanks appreciate ya

 

[Huckleberry]

Thanks for all the help. Keep ya posted

 

[duece1951]

1978 GMC K15. Stock crate 350ci. Mechanical fuel pump(stock). Does a fuel pump go weak or just quit? Truck sometimes starts hard but not always. Truck runs good, then lays down when throttle applied. (45-50 mph). Sometimes stalls out. Truck will restart. Replaced the distributor and Carb 1 year ago. I not sure sure thanks in advance.

By lays down do you mean the truck seems sluggish and surges about 45-50?

 

[Huckleberry]

Yes, as down....Boggs when gas applied. Not sure if it flooding with gas or starving for gas.... could the carb float be closing? Bad fuel pump? Vapor lock?. Changing the fuel pump as a start.

 

[duece1951]

I ask only because that’s the exact thing mine started to do at that speed. Ran rough as well. Fuel pump died while sitting in the garage. So we have the same issue with fuel delivery. I read that was a sign of them starting to fail. Thank you for the reply and makes my guess at the issue a good one.

 

[Ricko1966]

Yes, as down....Boggs when gas applied. Not sure if it flooding with gas or starving for gas.... could the carb float be closing? Bad fuel pump? Vapor lock?. Changing the fuel pump as a start.

Diagnose before changing parts. Your chances of getting a new good fuel pump is maybe 50/50. Check fuel pump pressure and volume. And the argument well its old anyway I'll just change it,okay let's go with that. 1/2 the new ones are out of spec right out of the box. So changing the pump didn't fix it,now let's change the carb,we'll that didn't fix it let's change the cap rotor and wires. Well now you have a vacuum leak at the carbureator base,a couple of crossed plug wires,a fuel pump making too much pressure, and a diagnosis nightmare because you added pieces to the equation.

 

[Sad Sack]

Always check fuel pressure and vacuum first if delivery is suspect. Get an inexpensive in-line pressure gauge and make sure delivery at idle is at least a steady 4lbs, throttle up to say 2000 rpms and what's your pressure.
Running vacuum, stock engine, at idle should be I believe a steady 21 hg.
Do you see black smoke when approaching your stall (bogging) point?
Have you checked hoses, carb screws, manifold nuts and choke pull offs? Filter installed right? FP leaking at weep hole?
Q-Jet are notorious for bad choke pull offs, loose screws, people putting in the anti-drain back filters in backwards.
Need way more info.

 

[Huckleberry]

Thanks for everything. No black or dark smoke. Carb is a edelbrock 1406. 1 year old. Did check vacuum and double checked the idle screws. I have not check the fuel pressure. Will do that. Replaced all spark plugs last Saturday. All wires good. Rubber fuel lines going into the pump look old .

 

[Sad Sack]

Thanks for everything. No black or dark smoke. Carb is a edelbrock 1406. 1 year old. Did check vacuum and double checked the idle screws. I have not check the fuel pressure. Will do that. Replaced all spark plugs last Saturday. All wires good. Rubber fuel lines going into the pump look old .

An Edelbrock 1400 series is what my truck has too and when trying sudden acceleration, it too bogs a little, but I turned it into the body shop right after buying it and hadn't much time to tinker with it other than readjusting the mixture screws using a vac gauge which improved it quite a bit. Some people swear by Edelbrocks as a plug n play carb, but my opinion is that they are "finicky" and once I get it back, I may have to consider Holley if fine tuning doesn't pan out.