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Old 04-20-2017, 12:18 PM   #1
Theoriginalsilverado
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Join Date: Nov 2013
Location: Norris Arm NL
Posts: 106
Question Fuel System Questions

Hey guys!

It's been a while since I've made a post. I just got home from university and I'll be going back on a work term soon, but before I do I was hoping to get my motor running. I have a bone stock 1975 Chevrolet C20 with a 350, camper special, regular cab long bed. I tore down the truck into all her parts, cleaned it all up and painted everything, replacing what needed replacing.

My question is about the fuel system. In taking it apart I wasn't as careful or methodical in dealing with the wiring and plumbing as I should have. I've managed to figure everything out without any issue, except for the fuel system. It has dual tanks, with the switch in the cab. I don't know much about the system and can't find much online.

I need someone to tell me what I need in place for the sending unit, regulator, and valve to work? I may be missing key parts of the wiring or simply not know what to do with it, so if someone could explain the whole system to me and what I need I'd appreciate it.

It's an embarrassing confession to make, but I'm self taught and have managed to figure out everything else, and I learn from my mistakes. I'd no doubt work this out on my own but I'm hoping to have it done in time before I have to leave for another year before I'll get to properly work on her again.

Any help is greatly appreciated! Thanks in advance!
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Old 04-20-2017, 02:29 PM   #2
Dead Parrot
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Re: Fuel System Questions

A lot depends on the emission equipment of the truck. My non-emission 75 with dual tanks vents to the air, doesn't have fuel pump returns and uses a simple 2 in 1 out solenoid selector valve. Others with more emission equipment can have a charcoal canister for the tank vents, fuel pump returns and a 4 in 2 out motor driven valve.

Do you still have the old sending units and or switch valve?
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Old 04-21-2017, 10:12 AM   #3
Theoriginalsilverado
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Quote:
Originally Posted by Dead Parrot View Post
A lot depends on the emission equipment of the truck. My non-emission 75 with dual tanks vents to the air, doesn't have fuel pump returns and uses a simple 2 in 1 out solenoid selector valve. Others with more emission equipment can have a charcoal canister for the tank vents, fuel pump returns and a 4 in 2 out motor driven valve.

Do you still have the old sending units and or switch valve?
I believe mine is the simple setup which simply vents to the air and has the 2 in 1 out solenoid you mentioned. I'm certain of it. So what does the system need to work in terms of wiring? Is there a pressure regulator somewhere in the system? What wires do the solenoid need and what wires do the sending unit need?

Thanks
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Old 04-21-2017, 01:39 PM   #4
Dead Parrot
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Re: Fuel System Questions

If yours is similar to mine, then you need one wire from the cab tank selector switch to the solenoid valve. Power on selects one side, power off selects the other. IIRC a pink wire.

Each sending unit will connect to a wire from the switch as well, IIRC a brown wire, should be two, one for each tank. A second short ground wire connects to the frame from each sending unit. You will need to match the sending wire to the valve position so you are reading the level in the side being used.

A short hose will go from the source pipe of each sending unit to the input of the valve. A 3rd short hose will go from the output of the valve to the metal frame pipe that runs up to the pump.

No regulator on the suction side of the system.

Use a short hose from each sending unit vent pipe for the vent system. Originally, the vent hose terminated in a plastic thing with a loose cap. Not sure that is made anymore. Some folks have used a rear end vent as a substitute.

Check wire colors from one of the wiring diagrams available on this site before proceeding. Verify with a volt/ohm meter as well. Who knows what a PO may have substituted at one point. And my memory of colors could be wrong.
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Old 04-21-2017, 03:31 PM   #5
Theoriginalsilverado
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Quote:
Originally Posted by Dead Parrot View Post
If yours is similar to mine, then you need one wire from the cab tank selector switch to the solenoid valve. Power on selects one side, power off selects the other. IIRC a pink wire.

Each sending unit will connect to a wire from the switch as well, IIRC a brown wire, should be two, one for each tank. A second short ground wire connects to the frame from each sending unit. You will need to match the sending wire to the valve position so you are reading the level in the side being used.

A short hose will go from the source pipe of each sending unit to the input of the valve. A 3rd short hose will go from the output of the valve to the metal frame pipe that runs up to the pump.

No regulator on the suction side of the system.

Use a short hose from each sending unit vent pipe for the vent system. Originally, the vent hose terminated in a plastic thing with a loose cap. Not sure that is made anymore. Some folks have used a rear end vent as a substitute.

Check wire colors from one of the wiring diagrams available on this site before proceeding. Verify with a volt/ohm meter as well. Who knows what a PO may have substituted at one point. And my memory of colors could be wrong.
That clears things up! I appreciate it. I just replaced all the fuel lines and the motor was just totally redone so all the lines are empty. Will the pumps be able to push enough gas up to the carb to get her running or will I have to try to get fuel in the lines myself? How would I go about doing that?

Thanks again!
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Old 04-23-2017, 07:22 AM   #6
Dead Parrot
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Re: Fuel System Questions

If your bed is still off, just run a temp piece of hose from the passenger sending unit to the frame pipe. Put at least 3 gallons in that tank. File the carb bowl through the vent if a Qjet. Get your motor running. Now connect your tanks, valves, and sending units the way they should be. If the motor quits running now, you know pretty much where the problem is.

If it will be a long time before you can work on it after getting the fuel system working, only put gas in one side. You don't want your next project to be dealing with two tanks of bad gas. Use your favorite variation on Stabil for the gas that will remain.

The reason for at least 3 gallons is the amount of unusable fuel in these tanks is about a gallon. At least that was what was in one of my tanks after running it until the motor quit.
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