Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

Can’t Lower Idle

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Chevynation:
Hello, I have a small block with a quadrajet (7040203) and the idle is around 800 so from time to time it may diesel for a second or two. It’s a 383 stroker with aluminum AFR 195 heads and 10.5 compression and runs on 93 pump gas. XR288HR cam. I have the total timing set at 36 degrees and 26 initial. I lowered it to 18 initial and it did lower the idle but I should be able to almost kill the car with the curb idle screw and I can’t. The car is a 4-speed by the way. The carb does not have a choke so there is not a fast idle screw to adjust. No vacuum leaks. The distributor is a MSD billet model but without the box and it has two medium advance springs. I have control over the idle mixture screws. The throttle blades are not open at idle. I was told that I’m getting too much air in from the bypass ports. How can I fix this issue? Thanks.

Chevynation:
Should I lower the initial timing? I’m using manifold vacuum by the way and I have 14 degrees from the vacuum advance. So I’m at 32 degrees at idle. The motor seems to like the higher initial timing with this cam and I don’t want it to develop any hesitation. Comp cams dyno sheets showed they had 16 initial and I have an 18 degree bushing in it currently but I have a 19, 21, 23, 25, and 28 degree bushing as well.

Kenth:
What is done to the idle tubes and downchannel restrictions?

Chevynation:
I’m not sure. Cliff rebuilt it and it runs great but that’s my only issue I’m fighting.

quadrajam:
Since You are sure the throttle blades are shut and it,s a Cliff build we can move
on to something else. I had a run-on issue once and blocked off the idle bypass air
and it didn't help a bit.
 Have you tried ported vacuum to the can?

 No accelerator linkage/cable bind? Brake booster vacuum leak?
You could check for a vacuum leak at the intake gaskets pretty easy by hooking
a vacuum gauge to the engine block with everything sealed off. There should be
ZERO vacuum. Pretty easy to do and something to eliminate.
Some cams can be very finicky to get to idle right. Good luck.

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