Can’t Lower Idle

[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.

[Chevynation]

I’ve tried disconnecting the vacuum advance as well and the idle does drop but I still can’t kill the car with the curb idle screw. No booster, carb, or intake leak. I’m not familiar with the method you’re describing for testing for a vacuum leak. Where do I connect my gauge to perform this test besides the ported vacuum port to display zero vacuum while running? Thanks.

[quadrajam]

This would be testing for leaks on the bottom sides of the intake gaskets that
are not accessible. You have to seal off the dipstick tube, oil fill hole, valve cover
breather hole and anything that exposes the crankcase to atmosphere.
I use that super sticky HVAC foil tape. degrease area before sticking it on.

Pull the PCV valve out of the valve cover and let it lay out of the way with
the hose still hooked up. The vacuum gauge kit usually comes with an adaptor
to connect it into the PCV grommet. otherwise you will have to get creative
with some fuel line and electrical tape. On a warm idling engine you should
see zero vacuum. not even a slight needle deflection negative.

Hopefully there will be no problem and you can tune it to compensate.

[Chevynation]

Interesting, I’ll be sure to try that out and report my results back to you. Thanks.

[Chevynation]

I covered the bypass ports and the idle did not drop any so I may have an internal vacuum leak. I left my gauge at home so I have to test for that tomorrow.

[Cliff Ruggles]

"So I’m at 32 degrees at idle."

That much timing at idle will make it difficult to slow the idle speed down.......

[Chevynation]

"So I’m at 32 degrees at idle."

That much timing at idle will make it difficult to slow the idle speed down.......

Ok I can take more initial out and let you know. I have to perform the vacuum leak test that was talked about yesterday.

[Chevynation]

Bad PCV hose. Changed it and everything was perfect. Thanks for all of your help everyone.

[Cliff Ruggles]

Good news!...