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 !!

[pfierb]

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 !!

My 73 2.0 did the same thing and no amount of adjusting and fiddleing ever stopped it.....Putting the FI system back on finally made the car run right and put an end to the backfiring.....Oh yes also got rid of the Pertronics went back to points.
Is that a picture of you as a kid sticking the knife into the wall socket if so how far across the room did you get blown?

[r_towle]

Maybe I did not see it, but is the backfire out the tailpipe, or out the carbs.

If its out the tailpipe, you are running to rich
If its out the carb, you are running to lean.

If you think its to rich, you need to research the jets, f-tube, and venturis.
Post those numbers.
Also, what Cam is in the motor...if you are using the stock cam, you will always have a compromise in performance, because the stock cam is designed to use with the FI system, and will not be completely tunable with carbs.
Saying that, you can tune the stock cam with carbs, its a combination of the correct jets and vents, along with changing the timing from stock.

Rich

[Joe Ricard]

Too rich on one of the two jets.

[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.

[rhodyguy]

if i read this right, you're running 7-8lbs of fp. that's too high for the 40s and prob adding to the rich condition on rundown/decel. if i were you i'd ditch the stock pump and the related regulating hardware, and get a self regulated rotary pump for carbs like the one offered by cb performance.

k

[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

[pin31]

Thanks Rich.

I'll purchase the book and write down all the numbers (I want to know and understand).

I will also order a carb sync tool too. Do I need to re-sync the carbs after I remove/clean/re-install the idle jets? Would a dirty or gummed up idle jet cause my rich running and backfire? Even above 3K RPM?

[rhodyguy]

in the side view picture, the idle jets are beneath the 2 brass screws adjacent to the fuel inlet, actually they are the jet holders. on the bottom side of the carbs the idle air mixture needle screws are the 2 center ones, they should have a spring on them. the 2 outer screws are the air bypass/bleed screws. note, there should be a little o-ring under the idle air mix screws. inspect them closley for cracks and deterioration.

in the top view, the main jet/ac/emulsion tubes are the brass items adjacent to the throats.

if you think you ned to blow out any of the circuits of the carbs, be very care with the air pressure you use. the little cans of compressed air work well for this.

the cb manual has a very comprehensive trouble shooting guide.

k