Mechanical fuel pump return line?

[joe dirt]

My cousin's new-to-him '86 K10 with a 305 has a dying fuel pump. He has 2 parts trucks, another '86 K10 with a 305 and an '84 K20 with a 350. All 3 trucks are carbureted. We were looking at his fuel pump last night and it has a third fuel line which I assume is a return line? Is the pump internally bypassed? We looked at his other 2 trucks to salvage a pump from them and they have the same mechanical pump with 3 lines. I assume the big line is from the tank, the smaller line is the return and obviously the metal line goes to the carb (through a new clear fuel filter we installed with 2 small pieces of flexible fuel hose)

Why does the pump I put on my '84 K20 only have the 1 feed line to the pump and the 1 line to the carb? Sorry, I've been into my Cummins diesels for 9 years now and just getting back into the gasser world. I didn't think carbureted motors needed a return line. Thanks.

 

[mrk5]

I know my truck has a return line from the fuel pump. My truck was orginally equipped with a diesel and I swapped a 350 in. I bought the fuel pump for an 84 K5 with 350 4bbl and the pump has a return line.

 

[pvfjr]

Sometimes vapor lock can be a problem with no return line. I had issues on my old 305, but I hooked up the return line when I put in the 350 and all is well. The gas can heat up from being near the engine heat for too long before it goes to the carb, and if the feed line is near the exhaust. Vapor doesn't exactly work properly in a float bowl, so needless to say, it starts to run like crap. The return line keeps fresh fuel recirculating through the lines so you don't get that problem.

 

[joe dirt]

Thanks. We're gonna swap fuel pumps this evening if the thunderstorms die down.

 

[joe dirt]

We GOT-R-DONE last night. The truck runs like a raped ape with the new fuel pump. Thanks