exhaust popping on light throttle and decel

Ok, I have searched and searched and searched for this very common problem that everyone seems to get at one time or another. Just looking for a little clarity...

I have a '79 308 w/ dual distributors and am only running the R1 points. Stock exhaust and cats.. carbs jetted with .55 idles, 135 mains, F-36, 200 a/c. Carbs sync'ed and pulling between 3.0-3.5 kg/h at idle. Idle set to 850-900rpm.

The car pulls very well and there is no stumbling during the transition from idles to mains. There is a little bit of a shudder when coming off of idle slowly until about 1500 rpm.

When I am at cruise and am light on the accelerator between 3000-4000rpm, I get an intermittent popping and "tweeting" sound through the exhaust (not through carbs). It is accentuated when I am completely off of the throttle at these same rpms and lower. This is not the "burble" on overrun that many think is "cool". This is more like the tweeting and backfiring sound where people look at you and say "Wow, you've got an F-car but can't afford to have it running right!"

It does pop a little through one of my carbs, but this is only while cold on initial startup. It goes away when warm.

I did just redo/recheck all the timing and replaced a HT wire that was getting a little bleed over causing a slight ignition problem. Also resynced and reset the Idle mixture screws and replaced two plugs (#1/#2) that were suspect. The mixtures "seem" to be ok now, with the exception of my #5 cylinder that I think is still a little rich. (See the pic below) The picture shows the eight plugs as you would be looking at them from the back of the car. The extra two plugs at the front right are the ones that I replaced, so the ones to their immediate left (#1/#2 cyl) are the new ones and still don't have the full color of the other plugs yet.

Also of note: when I pull the #1 and #2 plug wires while running, I am not getting ANY drop in rpms or change in the sound of the engine. Their wires are good and check to be firing with a timing light as well as checking to see that they are throwing a good spark (out of the engine). I checked the compression on these (and the other) cylinders. They are all running between 155-160psi. These numbers may be a little low, but I think has to do with me not having the right fitting to get down in through the cam covers and tighten securely. But the numbers were consistent across all cylinders.

Also, the #1/#2 cylinders are very difficult to set the idle mixtures on. All of the others there is a noticable drop in rpm both rich and lean, but those two cylinders are difficult to impossible to tell when running the screws in/out. I wound up setting them at about 3 turns and convincing myself that I heard "some" difference in rpm 1 turn either side.. I pulled their idle jets out and cleaned just to make sure, but I can still turn the screws in all the way without making the engine stumble....

Any thoughts? I am considering .60 idles, but I think that may be too rich. I have also considered that I may have blown out some baffles in the exhaust as another F-chatter discovered on his car....
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another picture. The first one looked a little dark....
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Carbs were recently rebuilt. dwell on the points is 34deg (as per WSM w/R2 disconnected). And compression was checked on all cylinders: 155-160 psi on all cylinders, no more than +- 3 psi variation. Timing is set for 7deg advance at idle (again, no R2 points) and 34deg at 5000rpm.

I do have one new coil. It was only really replaced to rule out a bad coil when trouble shooting an ignition problem. It had really no effect.... But, some of these popping problems really started bothering me AFTER this... Maybe I'll try the old coil again. But that still wouldn't explain the problem with the #1/2 cyl....

Part of me wonders if somehow the passages on the one carb (cyl #1,#2) are blocked to the idle mixture screws, preventing any fuel flow with throttle plates closed (no effect of idle mixture screws and the "shudder" on acceleration from idle).... Hmmmmmm....

 

New wires all the way around. Some more than once. Replaced #1/#2/#3 and rear coil wire a 2nd/3rd time.

New extenders all the way around.

No carbon tracing or problem w/dist. caps. One new one on rear bank.

 

the wires always show a hit from induction light. That is, according to my timing light (and my induction pen light), it is not intermittent on current

Also, pulled the wire and attached the plug, (out of the block) and grounded while the engine was running. Still had spark. Gap set at .027

 

That's a good idea. ( of course, being VERY CAREFUL while doing it) I will give that a try when I get back home.....

 

I initially had it around 900-1000, but took it down to a lower rpm. I wanted to bring it down lower to ensure that the throttle plates were as closed as possible to eliminate the possibility of fuel coming in from the other progression holes. (just wanted the bottom progression hole to be providing the fuel mixture in order for the mixture screw to be providing that fuel...)

I would be interested in getting some of the colortunes and trying those for tuning the mixtures. I have found a few sources, but would like to get the "professional" kit. (ie the one with four in the one kit) does anyone have a source for those???

 

Using: NGK BP6ES....

 

Ok. I tried this awhile back and discovered I was not getting sufficient fuel to those cylinders. I wound up pulling that carb and tearing it down again. I cleaned it throughly in solvent and blew all passages out with compressed air. I also noticed that there was virtually NO grease in the throttle shaft bearing. I believe that there was a bit of a vacuum leak through the throttle shaft and, possibly, some crud clogging a passage.

Anyhow, after cleaning and reassembly (and replacing the bearings w/ new sealed bearings), I can now set the mixture screws on this carb! They now have the effect they are supposed to have. And also, now when I pull the ignition wires to the cylinders associated with that carb at idle, there is the requisite rpm drop.

BUT, now I have a NEW problem.

When I tune the idle mixture screws to where they are supposed to be, I cannot get the idle rpm below 1500! I resync'ed the carbs and am pulling about 4.5kg/h through each barrel. The idle stop cable (mounted on the front cam cover) is completely loose and the idle set screws are all the way out, but still I get 1500rpm! It seems crazy that the throttle plates are completely closed and still I get 1500 rpm.

What did I do wrong?