Stubborn Lean Idle Problem

[71silverstar]

New member here...I have an original 7041267 from a survivor 71 T/A with 455 HO 4 spd.
I have rebuilt the carb with one of Cliff's SR rebuild kits and almost everything is running perfectly.
There is a persistent "miss" in the idle note and it hesitates in gear at idle and very slight part throttle settings.  I am absolutely positive at this point that it is too lean and I just can't quite get enough fuel moving through the mixture screws and the transfer slot.
Here's what I've done/know:
455 HO Bone Stock...no over boring, no cam change...true survivor just as it left the line in Van Nuys
SR Rebuild
New throttle bushings
Distributor professionally rebuilt and set to stock curve
Idle pickup Tube:  0.039"
Down Channel Restriction:  0.055"
Main Air Bleed:  0.070"
Upper Idle Air Bleed:  0.050"
Lower Idle Air Bleed:  0.067"
Main Jets: 0.073 (supplied by Cliff)
Primary rods:  38B (supplied by Cliff)
Vacuum at curb idle (1000 rpm):  15"
Initial timing (600 rpm, no vac advance):  14 deg
Hanger is dead level
Float at 1/4"
Mixture screws do not respond other than to stall engine when screwed in...once you back them out about 1 turn or so, the engine stabilizes and then backing them out further has little effect...maybe 50 rpm rise at best
Choke tip in has little effect either...as I close more of it off with my hand, it eventually speeds up then immediately stumbles
I have Cliff's book and have ordered new pickup tubes as I think these may be suspicious...I mean they're 54 years old...
Once I put enough throttle in to get past the transition, all is well.  Acceleration is smooth and steady, no pinging, no hesitation, no surging.

I just can't get the low speed (driving through the neighborhood) to be nice and smooth.  I know these carbs were ridiculously lean as originally built, so I wonder if today's gas just doesn't work right with the idle calibration.  My next thought is to try increasing the idle pickups to 0.041" and see how that compares.  After that, I would think I need to reduce the IAB's to try to improve the vacuum signal pulling fuel from the main wells.

Am I on the right track?  Anyone have any other ideas?
Thanks in Advance!

[71silverstar]

Something else I noticed that might be contributing...at base idle (600 rpm), the idle screw is barely touching the throttle valve (but it does make contact).  The transfer slots are not exposed, and the primary valves are fully seated (there is a very tiny amount of daylight visible around the top and bottom edge) 

[Kenth]

Is the adjustable main air bleed (off-idle adjustment) screw under the triangular washer seated?
Are the secondary throttle blades fully closed (centered)?

[71silverstar]

Yes, the adjustable air bleed is fully seated and the secondaries are fully closed (they fit perfectly actually).

[Kenth]

Before opening the idle tubes further i would take a hot cylinder compression test and verify the ignition system, running voltage to the coil, points dwell etc.

BTW, i have measured the idle tubes and downchannels in an unmolested 7041267 to .037" and .057" så i think you´re not to far off in that regard, but with todays fuels you may need .041" idle tubes.

[Cliff Ruggles]

.038" idle tubes would be plenty assuming stock engine with the original camshaft.

The 1971 455HO and 400 four speed carburetors are just too lean from the factory to work well on this new fuel. 

When you get time call me at the shop and I'll send a better jet/rod combo for it.  I've rebuilt and restored a lot of those and could never get one to pass testing with the stock tuning parts in it....

[71silverstar]

Will do...Thanks Cliff and Kenth!

[71silverstar]

Left a message at the shop...

[Cliff Ruggles]

If I didn't call you back then I didn't get  the message.  Happens at times as calls coming in drown out the message that's being left, or I wasn't clear, etc....

[71silverstar]

Sorry it took a while but I wanted to fill in the blanks on the final resolution.
As Cliff has said, most carburetor problems are actually ignition problems.
There was nothing wrong with the carb or the freshly rebuilt distributor either.
Cliff and Kenth suggested doing a cylinder pressure test just to make sure valves, rings, etc were not the culprit, so I bought a cylinder pressure tester and started removing the plugs.
Turns out, SOMEHOW, when I installed new plugs a while back, I managed to smash the electrodes down on 2 of the plugs so there was barely any gap and an obviously lousy spark.
I gapped every plug, was very careful putting them back in, and bam!  Fire breathing dragon!
And now, I have a solid 18+ inches of vacuum at curb idle, and the idle screws are responding.
I did replace the idle tubes with 0.038's.  I cracked the adjustable air bleed back off the stop about 1 turn as well.
It might be running a tad rich now, but that's just my nose telling me that after idling it back into the garage.
I can tweak a few things here and there now and see if I can really fine tune it, but it's going to be hard to improve on the performance right now.  It will spin the tires as originally designed, no problem.

[Kenth]

Well done!
Just a note: the adjustable airbleed on 1970 and later Pontiac Q-jets adds air to the upper main circuit air bleeds and does nothing for the air bleeds in the idle circuit as it does on 1969 and earlier Q-jets.

And, stinky exhaust is nearly almost the result of a too lean A/F mixture that's not completely burnt and is instead dumped out the exhaust pipes making your eyes tear and you clothes smell like garbage.

[71silverstar]

Hmm...that's interesting about the adjustable air bleed.
The stinky exhaust is not the funk you smell when the mixture is too lean...this is that raw gas smell...similar to sticking your nose over the carb after someone hits the accelerator pump.
In my quest to solve the "not enough fuel in transition", I may have overdone the jets/needle sizes and likely my idle mixture is rich.  I noticed the gas smell again in an open parking lot as I walked around the back of the car. 
Looks like I finally get a weekend to get back out and tinker, so we'll see what I find!