How many homebuilts fly "high"?

LeSving wrote:

…It has much more than nothing to do with temperature.

OK, perhaps, but in the test rig I mentioned I’m going to guess that the temperature in the lab was probably pretty stable and that the pressure drop in the fuel supply pipe to the pump was the reason for the vapour lock.

I had a 3.# litre deisel Ford farm tractor engine in a boat. The fuel pump had a glass cover. A blockage in the pipe from the tank reduced power, and showed vapour in the pump, which disappeared when the engine was shut down. I eventually found a wad of fibre in the inlet to the pump.
Red deisel has a much lower VP than Avgas, which is itself lower than the EN228 I buy from Tesco.