fuel sending units

[dalefan613]

so heres my question replacing both tanks on my 75 stepside c10 the sending units have 3 outlets my truck now has no vapor canister and when I went to a holley mechanical fuel which has and inlet and outlet no return like the stock so do I need a 3 outlet sending unit since I don't have a return line hooked up and I don't have a vapor canister can I use the one outlet sending unit  or am I creating a problem

[bd]

Well, the tanks used to vent through the vapor canister.  You can use a single tube pickup as long as you also use a vented cap.

[rich weyand]

You can just take a regular non-vented screw-on cap and drill a 1/16" hole through the center of it.

[dalefan613]

I have the old school locking caps on the stepsidenhow would I know if there vented  there not screw on

[DnStClr]

There are some vented caps on ebay for your stepside. Kind pricey tho.
http://www.ebay.com/itm/73-74-GMC-Fenderside-Pickup-Truck-C2500-C3500-Stainless-Gas-Fuel-Tank-Cap-/370880455181?_trksid=p2054897.l4276

[Pojimbo]

OK folks. Got a 73 Chevy. It's a 454 with a carb. Fuel pump has 2 tubes. One is smaller than the other. 
Got 2 new gas tanks, got 2 sending units with 2 tubes, and 2 sending units with 3 tubes. I'm putting dual tanks. Which sending units do I use?

Would like to know what the difference is with 2 vs 3 tubes??

[rich weyand]

2 tubes is supply and return.  3 tubes is supply, return and vent.  Back in the day, the caps were vented.  This let fumes into the atmosphere as fuel expanded and contracted with ambient temperature.  Third tube was added to capture fumes in a vapor canister, which was purged by being connected to engine vacuum, so the fumes got burnt through the engine.

If you go with 2 tubes, you need to use an older vented gas cap, or drill a 1/16" hole in an unvented gas cap.

[bd]

Three tube pick-ups have 1/4" (return), 5/16" (vent), and 3/8" (supply) hose connections.  Two tube pick-ups have either 1/4" (return) and 3/8" (supply) OR 5/16" (vent) and 3/8" (supply) hose connections.  On two tube and three tube sending units the 5/16" tube is generally baffled inside the tank to reduce liquid fuel in the vent line.  Measure the dual-tube pick-up units to determine which you have, but I rather think they are 5/16" by 3/8". 

The fuel pump you describe uses a 1/4" return hose and 3/8" supply hose, plus the 3/8" hard line to the carburetor.  The 1/4" return was used in some applications to help alleviate vapor lock during hot weather.  Under the circumstances I recommend the 3-tube pick-up units.

Whichever direction you choose, the tank has to be vented in order to draw properly.  So, you either need to vent the tank through the 5/16" hose or use a vented cap.