Morning Gentlemen. I have a quick fuel return line question. For context I have a '50 Pontiac. 350 Pontiac engine 700r4 trans. Pretty much my daily driver. A few times I have dealt with vapor lock in traffic so my plan is to hopefully end it. Im insulating the fuel line near any heat source but my real question is about adding a return. I have the carter p4070 pump mounted on the frame rail. I have at this point came on the outlet side of that pump with a filter that has a return. The pontiac tank has a drain in the bottom so I have ran a hard line and returned it into the bottom of the tank. What are your thoughts on having the return there vs up at the carb? If I understand vapor lock correctly the fuel "vaporizes" on the suction side of the pump due to temp and the fact its a liquid under vacuum. Id like to hear your opinions. If need be I can move the filter up near the carb. Thanks
Also check the tank vent for blockage. I had similar issues and a blocked vent line was the issue. Insofar as return line I'd move it to top of tank as fluid finds its own level relevant to how full gas tank is with return line on bottom
look at OEM systems, the return line comes from the engine mounted pump, or from the fuel filter near the carb. Heat is a big issue, vacuum not so much. Having a mechanical pump on the engine, as well as a "pulse" type pump near the tank, seems to get me through most of the tough situations on old cars. This lets the engine pump work most of the time, and can turn on the electric when dealing with going up a grade when it's hot out.
I wouldn't use the drain plug at the bottom of the tank. Even though you are using it as a return its still were all the **** in the tank is.
In 1967, the GTO's had the fuel filter right in front of the carburetor with a steel return line going back to the tank, from the filter it was about 8"s to the carb inlet. The '65s had the filter right above the fuel pump with the return all the way back to top of tank. The return should come off the filter as close to the carb as possible, this will move any hot fuel back to the tank. The return should also drop in the top of the tank, coming in the bottom will require over coming the weight of all the fuel trying to get out of the tank due to gravity, plus you don't want snag it on something and pull it off. Not sure what the gravity fuel will do when the engine is off. Dad and I have run Pontiac's since he bought his first one in 1955, vapor lock is pretty rare with them. Check your cooling system, air flow under the hood, and heat riser if it still has one. You didn't mention what carb, intake, or other modifications to the engines which may help shed light on the vapor issues.
OK. Tank vent isn't clogged. All I have in this car is the electric Carter pump. Tank is cleaned. By me and has been sealed so no debris in the tank. There is no chance of it ever getting snagged coming out of the drain because it's 90'd up and hardlined coming out of there. I understand there is a certain amount of resistance for the fuel to flow back in due to gravity. But what I really wonder is what temp does gas boil and turn to vapor at say 5 psi vs being under a vacuum coming from the tank? Also. I intend to move it up near the carburetor. This was just an easy way to do it for now just to see. I do understand that oem spends $$$$ to get things correct.
To solve , use the drain as return Fitting ,add a stem to the fitting so the weight of fuel not fighting the return , fuel will exit / return above fuel in tank , & maybe add little 90 deg L Or splice a return into filler neck / hose
As noted GM used fuel return lines on carbureted systems in the ‘60s. I did one on my 429 powered ‘49 Cad and it did solve the hot fuel “vapor lock” symptoms. Used a mid ‘60s Corvette fuel filter by the carb to provide the return line, 1/4” to the fuel filler pipe at the gas tank.
As @squirrel noted above, I handle vapor lock this way. SBC with mechanical pump. The Carter P4070 mounted near the tank with toggle switch. If car stumbles due to vapor lock I switch on the electric pump. Engine immediately continues to run. My guess is that the return line from electric pump back to tank will not prevent vapor lock, which most likely is occurring in the hot engine compartment, probably between the mechanical pump and carb. I also use electric pump to prime the carb when I haven't started the engine for a few days, or more.
In your original post it sounded like you were considering a "loop" return near the carb. I considered that at one time on a circle track car but was advised against that approach. If you do a return what you described should work OK although to the top of the tank would be more ideal.....less prone to a leak. As far as returning to the bottom of the tank the "back pressure" of returning against the weight of the fuel level in the tank would be really low......maybe 1/4 to 1/2 PSI since the depth of the tank is likely 8" give-or-take. Lynn
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