jtalka

Hi;

I need some help with my 1978 with the ZZ383. I'm running a Holley 4150 (3124 List, a service replacement from my old 396). Everything basically works fine with one small exception. I'm getting a flat spot in the carb right off the idle position.. I'll be cruising along, have my foot off the gas, everything is fine. When I step down, slightly, I get an initial flat spot (no acceleration) and if I push down more, I get past the flat spot and she will begin to accelerate. I assume that it is starving for fuel at that point...

What should I be looking at?

Thanks!

Joel

diehrd

Check the accel pump lever , my friends new in box holly had that arm bent just enough to make it stick slightly and it never rested flush with the cam that moves it.

jtalka

I did check that.. there was a gap in there which I adjusted out. Now I get instantaneous accelerator pump action...but still have the flat spot...

Joel

chevymans 77

Have you checked the transition slot adjustment?

this would be the first thing to check for an off idle flat spot.

try opening the idle adjustment screws between a 1/4 to 1/2 turn and test drive to see if the issue gets better.

Neal

jtalka

Neal;

I've tried opening the idle mixture screws without much difference if any. I did adjust the accelerator pump to make sure it moves as soon as the throttle is moved... Where is the transition slot adjustment? I had two of these carbs and wound up swapping the bottom plates so that I could create one dated unit with manifold vacuum for the VAC. That one went with the '65 396.. What's left is what I am using. Other than swapping, I did no adjusting...

Joel

toddalin

Flat spot? Maybe like this, or this, or this?







Your problem probably lies in the pump shot. Most people will say to put in bigger squirters. But as shown here, mine was caused by a rich condition from the squirt and the answer for me was smaller squirters and a different pump cam position. (Car Craft Magazine found the problem to be the same and also put in smaller squirters.)

To really get to the bottom of the problem, you need to put it on a chassis dyno with a simultaneous A/F read-out.

I can tell you that things are not always what they seem and you can't believe a lot of the literature (including Holley's own) that is out there.

MelWff

The 3124 is a vacuum secondary carb. Did you adjust the secondary stop screw 1/2 turn from completely closed. This generates minor airflow thru the secondaries and permits you to adjust the primary throttle plate so that it is in the correct position with respect to the transfer slot. When you adjusted the primary accellerator pump did you use a .020 feeler gauge and insert it between the pump arm and the lever on the throttle with the throtte held wide open? This gives you the proper tension on the spring without bottoming out the pump.

chevymans 77

Joel,

You'll need to pull the carb and turn it over to verify the transition slot adjustment.
It should look like a small square when adjusted right,
The second pic in the link is the transition slot but the one in the link is not adjusted right, to much of the slot showing.

http://www.google.com/url?sa=i&sourc...61908187130208

Once you have it adjusted correctly you may have to use the secondaries to get the idle right for your combo.

Neal

jtalka

No to both questions, but I will... Thanks for the information...

jtalka

You know, the more I think about this the more I believe that there may be a problem with the carb overall... I did switch throttle bases from another 3124 which I had. This throttle base provided ported vacuum and I asked the rebuilder to switch it over to manifold vacuum which he supposedly did. When I got the carb back the vacuum port, off the metering block, gave me 10"Hg at idle but the vacuum went up once the throttle was applied... Manifold vacuum for this motor is 15" at idle. This is obviously not manifold vacuum which should drop once throttle is applied. For my purposes, I plugged all the carb ports and tapped into the manifold directly..

MelWff

From the factory, the pipe on the metering block is ported vacuum and the pipe on the throttle body is manifold vacuum. A rebuilder cannt change this.