Thanks for these answers to your post! I just ran a can of Seafoam thru the carb on my 70 383 mag & whatta difference. The 383 sat on a stand for 7 yrs before the install in my 71 Power Wagon & had a big cam that didn't help matters. Low stall verter & tall 35" tires w/ 3.55's got the motor loaded up w/ carbon & suit for the past 8 mos. Poured 1/2 of the can down the carb @ 2000 rpm & then dumped enough to stall it. Let it sit for 15 minutes & fired it up. Smoked big time for a minute & then cleared right up. Truck runs much better & can notice more low end throttle response & smoother idle. Gonna switch the cam soon, but this was a good fix until then. Also put the last 1/4 in the tank.
BTW, I did put some Marvel mystery oil in the plug holes before the install to lube things up, but that was months ago. I also heard Marvel Mystery oil is good for a top end cleaner, too??

Issue still remains with engine idling at 1100-1200 RPM and running rich. From what I can see the carb is set to run lean (idle mixture screws are only 1 turn out from being bottomed out) and idle speed is set to lowest...

I am not sure if timing is my cuplrit here or maybe a vacum leak...motor seems to pull a consistant manifold vacum of 15 inches at idle...

you want to make sure the carb is on the idle circut and the curb idle setting is not opening the butterflys to uncover the transition slots makeing it rich at 1200 rpms.

use a timeing light to set base timeing,then use a vac gauge to adjust the air mixture screws to get the highest vac readings,then adjust curb idle,then recheck/adjust all 3 setting agian to dial it in.

if the air scews are that far in and its still fat,i wager it is on the power/crus mode and not the idle circut

I have my 440 dialed in to a 750 rpm idle timeing at 10*BTDC pulls 19 inches of vac

find what the engine likes with the timeing light and vac gauge and adjust acordingly

oh! and change the plugs,they could be fuel fould already,i had mine black and sooted up bad swaping carbs around one time and just need a fresh set of plugs.

Hello:
Think your on the right path by looking for a mechanical problem, but my experience with Holley Carbs is somewhat limiited. I have always used Carter (now Edelrock Carbs much easier to tune, in my opinion.) I'm sure someone here can help you on the Holley.
Engines are complicated devices and nothing you can pour out of a can will cure a badly tuned or improperly built engine.
That comment is not a reflection on you or your talents, just something to keep in the back of your head.

Maynard