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Old June 17th, 2008, 02:19 PM   #1
Familyman
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Default 76 390 Fuel Pump

I have to crank my 76 390 over quite a bit and pump it to get it to fire when it is cold with the manual choke on or off(edelbrock performer 600CFM). The choke is working properly so I started to think maybe it was the fuel pump losing pressure when it sits thus taking time when cranking to get it to fire after pressure is up where it should be. Does this make sense or am I having a carb issue? I started wondering about this when I went to NAPA to discuss the fuel pump and they said there was two pumps - one for th in cab tank and one for a frame mount tank. So I started wondering.... when I rebuilt this truck 10 years ago I switched from the incab tank to a frame mounted tank and did not switch fuel pumps at the time....could this be the problem?

Anyone?
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Old June 17th, 2008, 08:03 PM   #2
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anyone?
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Old June 17th, 2008, 08:26 PM   #3
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I can't say as I ever heard of a `76 with dual pumps. thats a new one even on me. I suspect your having a fule pump issue. either your pump is craping out or the filter is plugged restricting flow . I susgest to you to do a fuel pump test.

1. undo the line at the carb and put a length of fuel line on it to a 2 liter clear pop bottle.

2. have someone crank the engine over while you watch the pump "pulse" fuel into the bottle. make sure the hose fits loose enough to let out air as fuel comes in.

it should flow about 2-3 oz. of fuel for each "pulse". so it should fill about 1/4 of the bottle in about 10- 15 pulses. if its not moving fuel on that scale I suggest putting on a new pump.

by the way is the fuel cap on your tank vented? its an odd question but I was just thinking I have seen older systems stopped by adding a late model fuel cap for non veted systems.( it creates a vacuum in the tank againest the fuel pump)
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Old June 18th, 2008, 06:38 AM   #4
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It doesn't have duel pumps NAPA stated that there were two different pumps for the specific tank application and I switched tanks during my build-up. Interesting thought on the cap. Hadn't thought of that one. Went out last night and it fired up relatively ok about 3 seconds of cranking. It's been sitting since last week but it sometimes sits for weeks and even months then it is cranky cranky a lot. I should have pressure at the carb also when I pull the line off right? If there is no or little probably the pump?

Todd
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Old June 18th, 2008, 04:29 PM   #5
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yes the pump should hold fuel up to the carb in the line. if its leaking down then its just filling the line with air. once its full of air then you have to pump all the air out to get fuel flowing again. now if the pump is "sucking air" the fuel will be very foamy coming out. that is not good because it means theres leak in the line or the sender you have to find.
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Old June 19th, 2008, 10:16 AM   #6
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Well I picked up a pump from napa last night. The pump for the in cab tank had this huge sediment can on the bottom of it. I chose the other along with a new filter. I will let you know how it works. Hope it helps. Can't hurt.

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Old July 5th, 2008, 05:55 PM   #7
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my 79 f250 with a 460 and edelbrock carb does this same thing you describe. it has done this everytime it sets overnight. It is a fresh 460 a brand new carb, fuel pump, fuel filter, tank, and all lines have been replaced. I think two things happen when the truck sits. #1 is that the fuel in the carb bowls evaporates over time leaving less fuel for start up and #2 it that even a brand new mech. fuel pump is not capable of holding fuel in the line above the pump itself. I would imagine that there was nothing wrong with your pump, just that mid-70's technology can't deliver what we expect. With my truck after the fuel gets back to the carb and the truck starts i have no other problems with fuel delivery until I leave it set again. I don't particularly like having to use my starter to prime the carb everytime i want to use my truck so I have decided to install an electric fuel pump with the mech. as a back-up just in case. This will ensure that I get good fuel flow to the carb before i even hit the start button the first time. And i am sure that even the holley blue pump will not hold the pressure in the line after the pump is shut off but it is a lot easier on my conscience to flip a toggle switch to run the fuel pump rather than using my starter to roll over my well built BBF to activate the mech. pump.
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