mez

Helping a friend out 63 chrysler 300 413 long cross ram all stock restored car. has what appears to be factory carbs so I want to keep it that way I am goin to replace accell pumps as they only squirt when you hit it hard. I will check float levels etc while inside main question is in between the idle adjustment screws there is a large adjustment screw I have never seen before Its about 1/2 wide has a spring on it and looks just like the idle adjustments usually there is a vacuum port there. does anyone have experience with this and is it factory carb for this engine?

Coronet 500

Curiosity got me so I searched and found the answer. My first searches found this to be 1958-1963 Pontiac/Cadillac carb and called Idle Air Bleed Circuit.
The idle air bleed circuit



Many throw rocks at the AFB's with the idle air circuit (the large screw between the two idle mixture control screws).

I will admit that idle adjustment can be tricky with this system if the user is trying to burn E-10.

The solution to idle quality on "driver" vehicles (not numbers matching) is to install a conventional idle control. There is a "dimple" in the lower casting on the front drivers side (where the normal curb idle screw would be located). Drill through the dimple using a number 21 drill, and tap for a number 10 x 32 machine screw. Obtain a 10 x 32 machine screw and spring at the local hardware or auto parts store. Install the screw and spring. Screw the idle air screw all the way in, and then adjust the idle the conventional way.

If you wish to leave the adjust as the factory suggested with the idle air screw: for MOST applications, the beginning settings would be 1 turn on the idle mixture screws and 1 1/2 turns on the idle air screw.

Where these carbs are really useful is for those using big cams (low vacuum). The idle air screw, used in conjunction with the conventional idle screw is an adjustable idle bypass. Remember the holes drilled by the factory in the throttle plate on some of the performance carbs? No need to drill, as the idle air screw will do exactly the same function, and it is adjustable.

http://www.thecarburetorshop.com/AFBtuningtips.htm

mez

thanks thats very helpfull info I never knew they had that. I had noticed the idle speed setting screws were missing the throttle is closed but It was idling ok.