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 !!

[rhcb914]

You should also check for exhaust leaks. I have a 1911 with dual 40's and my old 1.7 stainless heat exchangers had steel flanges. I could not get a good seal to the muffler and I would get tons of popping when driving hard but not so much when I took it easy. I swiched over to a good set of 2.0l SSI and stock 2.0L muffler and I don't get any backfiring. The car runs great!

If it's running as rich as you say it should be easy to spot leaks. Just look under the car and see if you see any sooty black areas on the exhaust. Also check where the HE's mate to the heads another common place to leak. If you find leaks there I take a very long file and file both surfaces of the HE together. I have done this a few times and it has worked wonders.