Topic: Fuel pump on ’78 C3 350??
in Forum: C3 Engines
I don't think fuel pressure has much to do with a bog on a carbureted ride. When the carb bowl is full a float connected to a needle valve shuts off fuel flow from the pump.
I ended up sending my Q-Jet off to The Quad Shop in Rockford, IL for a total rebuild, 2 months and $345.00 later, It ran like a champ, still does......
Dave
Just got back from Advanced Auto doing some research. Turns out each fuel pump is factory set for a specific pressure setting. Mallory has one for 78's at 6-8 psi. and Holley has one for 78's set at 14 psi. I believe mine is set for 4-6, which is too low. For $65 I think I'll try the Mallory and hope that a little extra pressure will fill the bowl quicker. If not, then more work on the carb. Thanks, Wil.
You are about to piss away $65.00.... Then another $40.00 or more for a regulator to drop the pressure back down. Fuel pressure does not affect a carbureted mill. Higher pressure may turn into flooding.... and it's a pain-in-the-butt to replace the fuel pump.
Voice of experience.... I am running a regulated 3 - 4 psi fuel pressure to my Q-jet and no problems.
Look at Ken's post above then look into the acellerator pump on the carb first.
You didn't say what carb you are running.... I know an Edelbrook carb will bog if it has a spacer plate below it, may be true for a Q-Jet also.
Dave
Lifetime MemberModerator
4-6lbs is about right. Pressure is not a big issue here, but, volume is. If the float bowl is running out on hard accelleration(assuming the float level is correct), you should be looking for a restriction in the fuel delivery area. Kinked/collapsed rubber lines are common, and don't forget the "sock" filter in the fuel tank.
I have, in the past, run across a few cams that were badly worn on the fuel pump lobe, which creates a scenario of the pump not being fully compressed. It is not something that you would ever consider in the normal course of diagnostics, but it does happen. In this case, it will allow the pump to provide an normal pressure, but almost no volume once the float drops on a hard accell.
Joel Adams
C3VR Lifetime Member #56 
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Thanks Dave and Joel, your comments are helpful.
Last night I did another test, I think Ken suggested this. Put a tee and pressure gage in the fuel line between the pump and the Q-jet. At idle I have 3 to 4 psi. While driving at low rpm the pressure drops to about 1 psi. At acceleration the pressure continues to drop to less than 1 psi then as low as 0 and then the engine bogs. Now I'm back to "look for a restriction in the fuel delivery". Can the metering unit, pick up and screen be removed from the fuel tank without removing the tank? Is there a way to check and/or clean the "sock filter" / screen in place? I think Dave is right, I need to do some more work before changing the pump. Thanks, Wil
Lifetime MemberModerator
If the unit is in the bottom of the tank, it is fairly easy to remove.
Make sure there is very little/no fuel in the tank, first. Siphon as much as you can out. The unit is held in by a retaining ring that screws on. It has three/four "tabs" you can use to gently tap the ring, in a counterclock-wise direction. This will release the sender from the tank. (Remove the fuel lines first.)
Once you have the ring off, you can carefully manuver the unit out of the tank. The filter is on the end of the pick-up tube. Carefully twist the filter off, and replace. Then re-assemble the unit into the tank, fill with fuel, and enjoy.
Joel Adams
C3VR Lifetime Member #56
My Link
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"
Then again there's always the chance you got a bum fuel pump from Advance..... This same thing happened to me with my '78 a few years ago,,, and I fixed it... but can't remember what I did and how.... but I know I didn't have to drop the tank. This senility sucks!!!
Dave
Well last night I did more fuel line delivery tests with a pressure gage between the pump and Q-jet.
1) 5.75 psi at idle, warmed up, both with gas cap in place and with it removed.
2) Took the supply line off at the pump and installed a temporary fuel line from a clean 5 gal gas container to the fuel pump. Now the pump is up to 8 psi at idle, warmed up. Nice. 
I think this means Dave is right and the pump is not the problem.
3) I siphoned a gal of gas out of my fuel tank off the bottom to see if I would pick up any dirt or debris. Put it thru a coffee filter and there is no dirt, just clean gas. So I don't think the tank is loaded with rust or debris. Also, I never find anything in the filter on the carb. Is it possible that the pick up line is moving and pulling air?
4) I put some compressed air on the supply line pushing back toward the tank in hopes of pushing any clogs off the "sock" filter. Reattach the supply line and now have 6 psi.
5) Test Drive. Fuel pressure now responds correctly to RPM. As RPM goes up the fuel pressure goes up from min of 6 to a max of 8 psi. Sweet, no Bog. It's gone.
6) Another test drive just for the fun of it. No bog.
7) Next move is to get into the tank, replace the pick up filter and make sure the tank is clean.
Lifetime MemberModerator
There doesn't have to be any trash in the tank to stop up the filter there. Over time, fuel deposits will accumulate there, and turn to muck. I'll bet a new pick-up fliter will solve your problem.
Joel Adams
C3VR Lifetime Member #56
My Link
(click for Texas-sized view!) NCRS
"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"