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