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 !!

[pin31]

Thanks for the suggestions !!

pfierb: I'm not the kid in the picture (although I probably did that)

r-towle: backfire is from the exhaust (too rich). The engine was rebuilt with a cam for carbs (don't know which one). I've contacted the PO to get this info along with the jet sizes etc..

Joe Richard: could you explain? One of the carbs or one of the jets on one of the carbs? or both ..........

rhcb: I'll look for leaks. I also have the Triad dual outlet exhaust/muffler

rhodyguy: With the return line clamp backed off I should be around 3-4 psi (the way it is now). I've been thinking about ditching the PCU and stock pump and going with the rotary one. (even if it's not the problem).

Any other suggestions or things to try?????

[r_towle]

Thanks for the suggestions !!

pfierb: I'm not the kid in the picture (although I probably did that)

r-towle: backfire is from the exhaust (too rich). The engine was rebuilt with a cam for carbs (don't know which one). I've contacted the PO to get this info along with the jet sizes etc..

Joe Richard: could you explain? One of the carbs or one of the jets on one of the carbs? or both ..........

rhcb: I'll look for leaks. I also have the Triad dual outlet exhaust/muffler

rhodyguy: With the return line clamp backed off I should be around 3-4 psi (the way it is now). I've been thinking about ditching the PCU and stock pump and going with the rotary one. (even if it's not the problem).

Any other suggestions or things to try?????

Do you have a book on webers?
If not, get one.
Take off the air cleaner, remove the F-tube. There are two, and they are directly between the two large holes (venturis) for the air to go in.
These two tubes are approx 3 inches long and can be removed with a flat bladed screw driver.
Once you have removed these, there are numbers on each part.
this will be the number on the ftube, the air correction jet, and the main jet.
write those down.
Do all four, never assume that all the parts are the same, frequently they are not.
Now you will know these numbers.

Next, get a flashlight and look down the venturis, on the upper edge of the venturi you will see a number stamped into it, about 1 ich down from the top of the carb.
This is the size of the venturi, could be 32 could be 28...

write that down.

Now the last one is a bit hard to explain without a picture.
You are looking for the idle jet.
Its on the side of the carb on the outside of the motor...
If you look online you can find a diagram of the IDF 40mm carb and see where the idle jet is located.
Carefully remove one at a time, and be very aware if there is dirt on the jet, or the hole is clogges in any way...you need to learn how to spot it, and it could just be one idle jet that is dirty or clogged...

There is a number on the jet as well, write it down.
You will now have all the numbers needed to determine how to jet your carb...you need to know how its jetted now, to determine how to proceed.

Reading you plugs will be a very important part of tuning your car.

Once you are done, you wont have to do this anymore.
You will need to clean you idle jets if you park for any length of time and the gas has a chance to gum up the jet.

get a can of carb cleaner and inspect and clean each jet with cleaner...then re-install.

Rich