new 20 gallon tank

[Rough75C20]

Hello All,

Looking to upgrade my C20 Fuel system. I currently have a 14-gallon tank on the passenger side that is being used as the main tank. However, the sending unit isn't hooked up to the gauge so I'm guessing on gas level right now. I am looking at getting a new 20-gallon tank from Jegs. Question is about the fuel sending unit. the truck currently has a 305 with a 4-barrel Quadrajet. I'm looking at the fuel sending units and I believe I need the one with two outlets (supply and vent). I haven't seen a return line to the current tank. Is this normal for these old carb setups? I have attached what is currently in my shopping cart from Jegs. Let me know your thoughts. I'm planning on using a T with a ball valve to control which tank is being used, but the sending unit will be hooked up to the main tank.

[Captkaos]

73-87's only came with 16 or 20 gallon tanks, not 14.
Are you talking about adding an additional 20 gallon tank or replacing the existing 16 gallon tank?  Why not just fix the sending unit in the tank?
For the T/ball valve how are you going to do the Vent line?  I am unsure what the question is on the supply/send?  Can you elaborate?

[Rough75C20]

Hello,

Thanks for the reply. I believe the original tanks were pulled and whatever the previous owner had was put in on the passenger side, could be 16 gal. It is currently on the passenger side with the cap sticking out of the side of the bed. I would like to have a tank that actually uses the fuel door on the driver's side and my thinking was, I might as well get the 20-gallon tank since I have a long bed. So, I would bw adding an additional tank. Honestly, I'm not even sure where the vent line "vents" to. does this go to somewhere else on the engine? can it just be open or plugged off? Can the line be spliced into the vent line of the other tank? The question on the supply / send is there are two different sending units on Jegs for my year of truck. there is a 3 port and a 2-port sending unit. one has a return port as the third port, and the other one doesn't. I'm not sure how you know which one you need. I don't know if my truck needs a return line to the tank or if only needs a supply to the carb.

[JohnnyPopper]

Do a search here or elsewhere for fuel and vent line diagrams. You want to get this right or other gremlins will come to the party...

[Captkaos]

Generally speaking it vents to atmosphere, OR a charcoal Cannister/engine.  I would find out what you have on the opposite tank and make it match, being a C20 it is possible it only used the 2 port but I would check.
You don't want to vent one tank back into another tank.  Some of these also had vented caps, but no idea on the year.  You dont want to cap it off as the tank will pressurize.

[VileZambonie]

Your truck didn't come with a 305 so obviously someone swapped things around. Is the fuel pump one line in and one to the carburetor only?

[Rough75C20]

Hello all, thanks for the replies. I confirmed that the fuel pump is only one line in and one line to the carb. The only other line coming out of the top of the tank that I can see is just hanging under the truck near the fuel tank. I believe this is the vent line. So the current setup is only a two port sending unit from what I can tell.

[Rough75C20]

Can someone please post a picture of the fuel sending unit wiring at the front of the fuse box on the engine side? I'm trying to figure out which tan wire needs the sending unit wire connected to it but a lot of the wires on my truck at the fuse box have just been cut off. want to make sure I'm hooking it up to the correct wire. If someone could post a picture of the wire coming out of the fuse box that would be awesome. there is one on the left side bottom corner when looking at it from the front of the truck that looks promising but want to be sure.

Thank you!

[Captkaos]

Most all of the wiring schematics are here: http://forum.73-87chevytrucks.com/smforum/index.php?topic=30115.0
And depending on the year truck it may or may not be in the same place especially if it has been tampered with.

[ehjorten]

Curious...you mentioned getting the tank setup so it goes through the fuel door, but if your truck is a 1975 C20 like your tag, then you shouldn't have a fuel door, it should just be a fuel fill cap on the side of the bed. That is unless someone prior to you, swapped beds for a 1979+. I believe that 1979 was the first year for the fuel door.

[Rough75C20]

Thanks guys. I’m sure the bed was swapped at some point with something else. This truck seems to be cobbled together. It does have an older style tailgate with the “CHEVROLET” in the center.

To confirm if the wire is correct what is the best method? Check continuity between the plug of the back of the dash and the wires coming out of the fuse box? Does the accessory power need to be on to check this?

[bd]

75 C/K chassis single tank fuel gauge wiring (circuit 30, tan wire) used the firewall bulkhead connector cavity outlined in red below.

Dual tank wiring is more complicated.