Ideas on running a fuel return line

BillGrissom said
I am doing the same, but for a high-pressure fuel pump feeding a TBI. The fuel sender I recently got has a 3/8" outlet and 1/4" return. I am using the factory 5/16" tube for the return. It sounds like your sender is different since your small nipple was for a charcoal canister. Surprising for 1972, since I thought those started in the late '70's. Perhaps yours was a CA car.

Re vapor lock, interestingly newer engines (~2000+) don't have a return line. Pump bypass is done in the tank, with only a single tube to the engine. I understand that is an EPA reg to keep the gas in the tank cooler & limit boil-off. The higher pressures for MPFI (>50 psi) must prevent vapor generation in the tubing.

At least in Calfornia, charcoal canister usage actually dates back to the very early 70s.

When I moved to California in the mid 80s, I took my, from out of state, LA motored '71 Dodge van to a place just outside Sacramento for a smog test. It passed the sniffer with flying colours, but the guy doing the test told me the van had failed for missing some of the required smog equipment items, specifically the charcoal can and a preheat tube to the air cleaner inlet. After having spent the better part of a day at a pick your part yard and a couple more working on the van to install a charcoal canister and lines, wrapping the air preheat stove from a slant six exhaust manifold around a header tube to run the auto parts store bought paper tube up to the snorkle of the newly installed small diameter Buick V6 air cleaner disguising the 340's four barrel (and making it look much more like the original two barrel 318 that had once resided there), I was more than a little incensed. I told him get his lazy *** under the rig and look, and not to assume that just because it had headers that the stuff was missing.

Needless to say, the smog test was passed with no more drama and nary another word said.