Holley 4150 Flat Spot
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Holley 4150 Flat Spot
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
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
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Joel
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From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
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
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
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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
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Joined: Oct 2000
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From: Santa Ana CA
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.
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.
Last edited by toddalin; Feb 25, 2013 at 01:36 PM.
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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.
Melting Slicks
Joined: May 2002
Posts: 2,716
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From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
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
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
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
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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.
Thread Starter
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Joined: Jun 2011
Posts: 80
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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
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