What does this screw do? carb question...

[digimurda]

hi all,
I've been just trying to learn more about my cars quadrajet carb, would anyone know what's the circled screw on the picture for and what's its purpose?

 

[vanrah]

Greetings Mr. Digimurda & all; I haven't fooled around with an electric Q-jet & it's been years (1986) since I've seen mine (a 1979 unit), it's in a box. But if memory serves, you'll find a plunger (piston) & spring controlling your main jet metering rods. And the spring is controlled by vacuum, & that is one factor that makes Q-jets more complexed to tune. If you change any thing that affects the vacuum signal you'll have to change the spring, rods & or jets. And the secondary jets also have rods controlled by the air valve (flapper). And the flapper is controlled by the spring & set screw at the end of the shaft. Also the secondary rod hanger comes into the equation. Hence all these adjustments make the Q-jet a fine fuel metering unit, but damn hard for the shade tree mechanic to tune. One of the many reasons so many of us hot rod type guys run a Holley, their easy. I personally have three books devoted to Holley's & I can actually carry on a conversation about Holley's & you won't think I'm a fool. So I'll bow out now & leave this thread to the Q-jet guys as I'm in over my head. Bob Jr.

 

[Bonnewagon]

I've only done one CCC Q-jet and it was a failure. For the life of me I cannot remember what that was for. Sorry, someone else will surely know.

 

[pontiacgp]

I have no idea either but came across this form that may help
http://www.corvetteforum.com/forums...computer-controlled-quadrajet-adjustment.html

 

[Clone TIE Pilot]

The screw is the idle air bleed valve (IAB) adjusting screw, it is for fine tuning the idle mixture for your Qjet. You hook up a dwell meter to the green plug on your computer harness, and adjust the IAB to set the idle mixture dwell. It sounds hard but it is quite easy. Two key things that can screw noobs up are that the idle mixture and idle speed must be adjusted in Drive and to check the reading after you put the air cleaner lid back on. If the reading is still off, pop the lid, adjust again, and close the lid and cheak the reading again.