Whoa.....why in the frack did they not make this standard on all Holley's?
Never seen this before....learn something everyday!

Jebby

 

are you aware they did that on certain Quadrajets? I have a 71 pontiac GTO, 455 HO Ram Air. On the top front area of the air horn there is a small triangular clip. If you remove the clip there is a small screw at the bottom of a shaft that performs a similar function. I only discovered it after Cliff Ruggles mentioned it in a online thread.

 

Very interesting information. The carb I have must not be OEM since it doesn't have these air bleed screws but rather has the brass insert air bleeds.

 

No I was not! Probably why that carb is worth $3500!

Jebby

 

Ok so it is not factory line installed. Probably an over the counter version. or worse...
The "162" date code corresponds to 1971, June, 2nd week.
Does that correspond to the build date of your car?
I would also check all the casting numbers, metering block numbers, etc and make sure they are correct.
Past that I would check jet numbers and power valve numbers for correctness.
The next thing I would check is that orifice size. IMO it looks the same size s the one for my 454! Why that large?
You could (carefully) insert a couple of pieces of copper wire into that air bleed to restrict it and see if the mixture screws magically start working again.
Shoot carb cleaner on the throttle shaft to check for leaks.
There must be a reason your mixture screws have no effect!
Take the carb off, flip it over and look at both transfer slots. They should look square, and even front/rear, not like a slot at all, at idle.
There is a hidden secondary throttle idle screw that you can only see when the carb is upside down. It is used to set the rear transfer slots.
Plug all vacuum ports for testing.
Keep at it and you will find it.



This is a generic 4781 850 DP I found on the net. The low speed air bleed insert orifice size looks more like what I remember on most Holleys.

My Question is "Why are yours so large?"

 

Update: I ended up adjusting the valve lash. They were a little tight but that didn't make much difference. Without getting too detailed, I focused on the carb and finally got it to where the idle mixture screws were adjusting as they should. The final problem I found on the carb was the primary metering block to body gasket. Although it was new, fuel was finding it's way into the vacuum recess for the PV. Vacuum at idle is now at 13 inches and the engine is running good. Thanks again for all the replies.

 

Terrific!
Problem solved. Mixture screws work. Vacuum sounds about right for an LT-1.