[JLMounce]

Most common reason with the FiTech for such a situation is a vacuum leak, followed by timing issues.

The 2 psi jump in fuel pressure from 58 to 60 is fairly inconsequential, especially considering a backfire state.

What fuel system are you running?
What model FiTech?
What ignition system is being run? Is the FiTech controlling timing at all?

This combination has a lot going on. A RA4 cam in a 301t with a 72cc head is a massive cam. Large enough that even the cam 4 profile in the FiTech may not be lean enough at idle to cope.

Have you been able to run the engine long enough to get an idea of what type of vacuum the engine pulls? The FiTech works quite well down to about 7-8” Hg. Below that it requires some heavy handed tuning to get it to run right. There are people that have these running on combinations making as low as 2-3” of vacuum, but the amount of work necessary to do so puts you in a position that running a Holley HP, Haltech 950 or Fueltech 600 would have been easier.

What occurs on these systems when there is a very large cam in use is that idle fueling can be rich enough that raw fuel is passing through the combustion chamber in the overlap duration and showing the oxygen sensor raw fuel. The ecu reads this as a rich condition and pulls fuel, creating an oscillating rich/lean condition. It’s possible this oscillation could grow large enough to cause a lean backfire. If you’re not on map 4 to begin with, try that first. If you’re already using map 4, you’re going to need to break out the laptop, load the tune in procal and edit the VE table to further reduce idle fueling.