[Dave Murdoch]

I've read many posts on this topic and still having trouble diagnosing this problem. My '69 350 stumbles as I accelerate just off of idle. Runs smooth throughout the rest of the range, just seems sloppy on the bottom end. I'm running a pretty mild cam (218 deg and 0.454" at .5) with an edelbrock 600 carb. Timing is set at 14 degrees initial on the timed vacuum port. I'm getting 14" of vacuum at 750 rpm off of the manifold vacuum port on the carb.

I've tried moving the accelerator pump to the hole giving the biggest squirt and can't tell much if any difference. I tried running the distributor off of the manifold vacuum and no difference. Edelbrock support suggest going to a richer metering rod. Does this sound right?

Everything I've read about this problem is related to big cams and low vacuum, but this doesn't seem like it should be the case with my setup. Does idle mixture have any effect on this? I set the idle mixture to maximize the vacuum.

If it matters, the rest of the set up is headers, performer RPM manifold, stock heads, 9.6:1 static CR, TH 350 and 2.56 rear end. Any ideas are much appreciated. Thanks.

 

[Pat Kelley]

Two things caused a slight roughness just off idle on my car. First, the coil (HEI) was questionable. Changing it helped. Second, increasing the idle mixture an tiny bit solved it. If this will help your problem, I don't know. Also check for vacuum leaks. If this is on a Performer Plus or other spreadbore mainfold, you need the adapter plate. It insures that the carb is sealed at the manifold.

 

[Got_CID?]

Do yourself a favor and buy one of their calibration kits. It comes with about 4 different pairs of metering rods, jets, and different step up springs for the metering rods. Just changing the springs in my carb helped quite a bit, and from there I went up to a richer metering rod until it got worse. The kits are around $35 but allow you to try numerous combinations until you find one that works right for you.

 

[kerryj68]

Besides the above you can try a new squirter kit from Summit #EDL-1475. It has three different sizes. The cluster has a size stamped on top. Try one size larger. It got rid of my hesitation.

 

[Brob]

I’m running a Edelbrock 1405, 600 cfm carb. Before I started the engine I had Dave "The Ignition Man" do his mod on my HEI . Dave told me ZZ4’s like to stumble so I should reach full advance around 2000 RPM. He also suggested running on the non ported side, which I do. I set TDC set at 11º static. I’m about 31º full advance.

I don't have very many miles on the car but it runs real strong.