[421Bonneville]

One of the many different versions of the AFB 4 barrel has a large screw between the two small mixture adjustment screws in the bottom front of the carb. What the heck is this? Can't find a reference to it in Chilton, Motors, etc.

[machine shop tom]

That's the idle air adjustment screw, used on some Pontiac, Cadillac, Chrysler, and Linclon models. It is used to adjust idle speed. With the engine at normal operating temp, turn the idle air screw outward to increase engine speed. This will lean the engine out which must be compensated by re-adjusting the idle mixture screws.

(info from 1969 Motors manual)

tom

[Pony]

Found it in a 1973 Motors Manual in the Carter carb section. Says that type of carb was used on Pontiac and Lincoln. It's an Idle Air Screw. States that this carb doesn't have the usual idle speed screw and uses this for that function. You turn the screw outward to increase idle speed, this leans out the idle circuit and the two idle mixture need to be adjusted at the same time to get the idle speed and mixture correct.

ps. Looks like Tom and I found it at the same time

[machine shop tom]

Great minds think alike! (And look up stuff at the same time, also!)

tom

[421Bonneville]

To the great minds, thanks from those of us with less than great minds. Frank