Need help with a carb question !, backfiring when I let off the accelerator Options

[pin31]

Here's the data:
PO owner modifications:
1974 2.0L w/dual weber 40s
stock fuel pump with pressure control unit (w/return line clamp backed off).
pressure control unit = guage with a clamp on the return fuel line (PO installed for autoX). The idea is to restrict the return line to increase pressure from street driving 3-4psi (clamp backed off) to 7-8 psi (clamp restricting return line).

My modifications:
Removed points and installed Pertronix II electronic ignition.
Have not changed/adjusted the timing

Observations:
Car starts fine, idles great (after initial 5-10 sec of throtle play when starting cold)
Car drives great.
Plenty of power in all RPM ranges.
Running a bit rich (plugs are black)
The car backfires when I let up on the gas (when driving more aggressivly in higher gears) not so much (or at all) in 1st or 2nd or when I'll taking it easy.

Because the car runs great (except for the backfiring) I did not want to start blindly messing with the carb adjustments if it could be something else.

Could it be:
carb mixture?
floats?
pressure control unit?
stock fuel pump instead of lower pressure carb pump?
timing?
etc.......

What do you suggest ????

Thanks !!