Eventually my Mighty seemed to change
its ability to hold a tune.
Like Wildman said, my Idle Ez had no effect whatsoever, although it did when brand new, and despite another tear-down and a very thorough cleaning, the carb began to act up like a Holley will when the air bleeds get dirty. I was never able to discover why, but it should be said that I did not go so far as to re-machine the castings. Privately I always have suspected that the finished/machined casting had never been cleaned very well and there were chips present in some of the passages.
The customer service at Barry Grant sucked. Not my opinion, just the result I got whenever I asked for help. Strictly a clinical observation that I am reporting here.
In the end, the carb ran so lean that it constantly bogged, but laughably it got very good mileage once you got up to a steady speed due to the overly lean mixture.
Adusting the idle mix would send it rich almost immediately, no matter which of the 4 you would turn.
The original cam I ran with this carb was the stock ZZ454 roller, a pretty mild cam that idled at 15-16".
Arguably, it could be said that the 850 might be too large for a mild-cammed 454, but the intake was an Air-Gap and that plus the head/valve combo contribute to great signal at idle in these engines. The Mighty had Annular boosters, which also would tend to compensate for a too-large venturi at idle by cutting down some of the cfm due to larger booster size, and the increased sensitivity and atomization of that booster design.
I don't subscribe to this, however. Properly tuned, which means using the correct intake manifold and proper timing curves, the 850 really is not too much cfm for a 454 with mild or greater performance build.
I too had to crank down on the float bowl screws to the point of worrying whether I might have distorted the main body in order to get them to stop leaking.
If I were to get another BG carb, I'd treat it as a kit, one which should be viewed as owner-responsibility to clean, measure, assemble, and tune.
As an aside, I found the DVD which accompanied my carb to be utterly worthless in tuning or modifying for a purpose.
Therefore, since I am an independent filmmaker, I produced a 45-minute film going into great detail on the teardown, cleaning, repair, theory, and rebuilding of a Quadrajet carburetor, just to see if it could be done. No window dressing, just hard facts and a complete teardown and rebuild, close-up, HD quality, complete narrative explanation, etc etc.
ARE YOU LISTENING BARRY GRANT?
Here's my Mighty on my ZZ mill. It's now injected, and I love it.