Even after years of working on or beloved LBCs, you can still overlook something and get bitten in the rear...
A recent job involved bringing a long-stored car out of hibernation. The car had not been run in many, many years. We did the brakes, chased down some wiring issues, changed all the fluids, and replaced the crusty fuel pump with a new electronic SU. The carbs were in fine shape. We drained the fuel out of the tank, and while it was a bit stale, it was nowhere near as bad as I had expected.
We tested the system, and after adding fresh gas, we primed the system with good, clean fuel. The car lit off and ran like a top while in the shop. However, whenever we tried to to run it on the road, it would starve for fuel and die. We found that the new pump was getting hot. Must be a bad pump, right? We swap out a new one. Same deal. This is not right.
Stay with me.
We had fuel coming out of the tank pickup, yes, clean supply filter, yes, flow at the carbs, yes, good voltage at the pump, good ground, yes.
I was finally able to re-create the heat issue on the bench by blocking the fuel inlet to the fuel pump with my thumb and holding it for several minutes. If you understand the way an SU pump works, the magnet is only energized for a breif moment, while pulling the diaphragm up. Once the diaphragm reaches its travel limit, the magnet shuts off and the spring presses on the diaphragm to make pressure. SU pumps cannot pull, they can only push. Therefore, blocking the inlet caused a vacuum to form on the inlet side, slowing down the diaphragm and keeping the magnet energized.
Working backwards, we could only surmise that there was a problem in the fuel pickup in the tank. We could even blow through the pickup outlet, but there was a slight restriction. Bear in mind that draining the tank was the first thing we did, since we assumed that the tank would be rusty and crusty. The fuel came out clean and only slightly stale.
