I'm considering replacing the current fuel tank sending unit in my '64 Skylark with one that incorporates a 1/4" return line connection. I'll be using a Mallory 4110 electric pump with the 4309 regulator so I can keep the fuel pressure set to a level that keeps the Q-Jet happy (the pump is set for 7PSI which may be a little high for the Q-jet). The 4309 regulator has provision for a 3/8" return line but I'd like to reduce it to 1/4" so it's compatible with the return built into the sending unit. Is this acceptable at that fuel pressure level or should I get a single line sending unit and braze a 3/8" return line fitting onto the tank (I'm replacing the tank as well)?
I tried a similar setup on my 71 and found the 1/4" return could not flow enough fuel at idle for the regulator to work. It would just get backed up and the fuel pressure would jump way past what the q-jet needle could handle. You have to remember, at idle the pump is trying to push the same volume as it would be at full throttle but all the fuel is going to the return line instead of the engine. I ended up using a 2-port non-bypass regulator.
Im using a 4140 and the 4309 and the directions specifically state that the return must be atleast the same size as the supply but its recommended to be one size larger.
Generally you want to run a return line either equal to your feed or only one size smaller at most. My set up runs a 3/8th mechanical pump to a surge tank, then a 5/16th to the fuel rails and a 5/16th back to the surge tank and fuel tank. How ever my primary 3/8th feed being mechanical is pumping less volume at idle than at higher rpm. IF I were running a 3/8th electric pump from the tank, I'd probably need a larger return line.
