Idle Surge/Starvation/Miss/Vacuum Leak

[Steeveedee]

OP, you checked for vacuum leaks. Did you squirt the primary throttle shaft on both sides of the carburetor? If the throttle body is worn enough, the primary blades can get enough off-alignment to cause high idle.

[garyd1961]

Edited.

[dz1087]

Originally Posted by Steeveedee

OP, you checked for vacuum leaks. Did you squirt the primary throttle shaft on both sides of the carburetor? If the throttle body is worn enough, the primary blades can get enough off-alignment to cause high idle.

I'll giver her a squirt later today and check on that. The carb was rebuilt 8k ago by Quadrajet Power. They didn't say anything was worn out beyond spec.

Originally Posted by garyd1961

I didn't see where you said what the compression ratio of your motor was but one thing I have learned over the years is you don't want to run ported vacuum on a high compression motor. With ported vacuum you increase the timing while accelerating which can cause spark knock. With manifold vacuum you increase timing at idle and cruise which is best for a high compression motor. With manifold vacuum when you accelerate the vacuum advance drops out the equation and you run on initial advance and mechanical advance.

The only difference in ported vs. manifold is that the vacuum signal at idle with the primaries closed is zero. Once the primary blades are cracked slightly, the distro sees the exact same vacuum signal that it would on ported. Ported absolutely does not increase advance, or vacuum as you open the primaries or secondaries. It goes to zero, just like manifold.

On my engine, the difference between the two schemes are as follows:

Manifold: 20 in/hg, then decreasing to zero as the blades are opened fully.

Ported: 0 in/hg, then ~20 once the blades are cracked slightly open, then decreasing to zero at the same rate as manifold as the blades are opened fully.

I've verified this with both my vac gauge and my timing light.

Otherwise, I'm running a stock rebuild with dished pistons and 883 heads. I'm definitely not high compression. I'm going to continue to check for leaks and adjust the choke mechanism on the carb to ensure it is working properly. Once I messed with the idle mixture screws, my idle settled really settled out.

[Steeveedee]

Originally Posted by dz1087

I'll giver her a squirt later today and check on that. The carb was rebuilt 8k ago by Quadrajet Power. They didn't say anything was worn out beyond spec.

Mark would have put bushings in if the throttle was excessively worn. So probably OK, but good to check. Looks like you got 'er licked, anyway.

[garyd1961]

Originally Posted by dz1087

I'll giver her a squirt later today and check on that. The carb was rebuilt 8k ago by Quadrajet Power. They didn't say anything was worn out beyond spec.



The only difference in ported vs. manifold is that the vacuum signal at idle with the primaries closed is zero. Once the primary blades are cracked slightly, the distro sees the exact same vacuum signal that it would on ported. Ported absolutely does not increase advance, or vacuum as you open the primaries or secondaries. It goes to zero, just like manifold.

On my engine, the difference between the two schemes are as follows:

Manifold: 20 in/hg, then decreasing to zero as the blades are opened fully.

Ported: 0 in/hg, then ~20 once the blades are cracked slightly open, then decreasing to zero at the same rate as manifold as the blades are opened fully.

I've verified this with both my vac gauge and my timing light.

.

Never too old to learn, I believe you are correct. I got my erroneous information from a supposedly top mechanic of a hotrodder forum that I was once a member of. The way you explained it makes more sense than his explanation.
ETA, I concede to what you say about port vacuum not raising timing advance under acceleration. Although when running vacuum advance I always run manifold vacuum and it seems to be the best from my experience. From what I have read port vacuum was an early attempt at pollution control by delaying the burn at idle so the exhaust gas would still be burning when it entered the exhaust manifold. This and very little initial timing helped reduce pollution but also killed performance. Manifold vacuum gives a better idle.
Saying this I don't have vacuum advance on my vortec 350, not that I think it is a bad thing, I just had a distributor without it so that's what I used. I get away with no vacuum advance by upping my initial advance and limiting my mechanical advance. Not the best for a street motor but it works for me.