3/8 vs 5/16 fuel lines
Another issue with running two pumps is one can give you X psi and then the second pump will raise x's pressure some more. Could the OP being popping his float needle valves and actually flooding the engine when it won't spool up?
Issue with cars that take a lot of cranking to get started off a mechanical pump... is sometimes the pump check valves are stuck open.... and the fuel drains back to the tank.
Stock mechanical pump only. One car is a properly working thermostatic choke and other is mechanical. Choke, tap, key, run.
Another issue with running two pumps is one can give you X psi and then the second pump will raise x's pressure some more. Could the OP being popping his float needle valves and actually flooding the engine when it won't spool up?
Issue with cars that take a lot of cranking to get started off a mechanical pump... is sometimes the pump check valves are stuck open.... and the fuel drains back to the tank.