Why do you want 4psi ? SU's require virtually NO pressure to work.3.2psi is perfectly fine, I would even say 4.2psi is bordering on too high, especially if needle & seat aren't in 100% condition. Fuel pressure is definitely NOT causing a backfire from the intake, something else is not right to cause that.
Have you checked everything else in the fuel system ? Are you sure you don't have any minor restrictions / blockages / kinks / etc (anything that may restrict flow between pump and carbs) ? Your pump may be putting out 9psi, but that 9psi isn't making it to the carbs ? Have you tested output from the pump at the pump, not at the carb after running it through near 50 year old plumbing ?
Regardless, you don't need 4 psi or even 3 psi, I guarantee you I could get an SU to run perfectly with 0.1 psi.
Edited by 1600dave, 08 March 2018 - 08:47 AM.