HOLLEY STREET AVENGER RUNNING HIGH

Have you checked timing? You might need to back off the timing a bit.

That detonation at shutdown you speak of is called "run on", that typically happens when the throttle blades are open too much over exposing the idle transfer slots giving the engine extra fuel.

If everything was set perfectly before, sounds like the primary throttle shaft is sticking and the return springs are not strong enough on its own to set it back to the actual idle you set, take the carburator off and spray carb cleaner into both throttle shafts just to be sure.

I currently run the same Holley 670SA on my ZZ4 crate engine, but its the earlier version with the idle mixture screws on the primary side metering block only, its a decent carburetor once you've figured out all the bugs, lots of complaints over that one and the Holley 770SA, of which i also have thats the earlier version as well, it came with the car, the engine seems to be much happier with that one, you're 406 would benefit IMO with a Holley 750, vacsec for auto, mechsec for manual transmissions.

Run on issues as stated ... Check that.

If it is actual detonation, first question is distributor hooked to manifold vacuum or venturi vacuum? If it's at manifold vacuum try switch to ported (Venturi) vacuum.

As engine slows down, plates shut, manifold vacuum increase, vacuum can gets stronger signal and increases base timing.

As engine slows down, plates shut, ported vacuum signal gets weaker and decreases base timing.

One scenario eludes to air/fuel issue, the other eludes to a timing issue.

Next to that is mechanical issues.... Sticking throttle plates in the carburator, and/or sticking vacuum controlled timing actuator in the distributor.

That's the simple answer. Holley is different from Q Jet, yet both still troubleshoot basically the same. Q Jet run on is usually throttle plates are excessive open at idle, Holley is usually transfer tube o ring fail. ...Actually this is more and issue of "loading up" and not "run on" with the Holley... But result is basically the same.

The other issues with Q Jet is the primary throttle plate bore is worn out and becomes oval shape. This allows excess air into the A/F stream and can cause the run on symptom when trying to shut down.

Make sure plates are shut, maybe turn out primary idle screws about 1/4 to 1/2 turn each and check base timing.

Ernest said:

.....Your 406 would benefit IMO with a Holley 750, vacsec for auto, mechsec for manual transmissions.

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Definately... Even though I like the Mech Secondary Holley for even Auto Trans.

Even so, trouble shoot what you got. You know it was correct before.

The only thing that changed in the process was the ambient Temp. Hot weather reduces density of fuel and air... Cold Weather increases intrinsic density of fuel and air. As as far as known..., was the only change. Therefore: whole issue seems to be an A/F mix or volume issue.

dickeymoore said:

I backed the timing down

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Let us know the outcome. It's a quick/simple thing that can easily be moved back if there's no difference. Whatever you do, try & make only one change @ a time to help isolate your decision tree.

78Delat88 provided some great suggestions as well.