Timing issue

[AR67GTX]

Nothing wrong with 9.6:1 compression, especially with aluminum heads. I suspect you are running a Holley carb? If so are your idle mixture screws responsive to adjustment? It could be that the front throttle blades are open too far and exposing too much of the transition slot, pulling in transition fuel at idle and producing a rich idle and too much air past the blades to allow the engine to shut off from idle when the kill is turned off. I won’t go into how to correct for that as it’s just speculation that you have a Holley and the front plates are opened too far.

 

[1969CoronetR/T]

Yes it is a Holley and the shop did tell me that the idle was too high (lowered) and that the throttle plates would not close completely.

 

[AR67GTX]

It’s a bit of a pain but I would pull the carb off, drain the fuel, and turn it over upside down with the primary throttle plates closed to the curb idle position (fast idle off) and examine the transition slot at the front of the primary bores. You should only see a square showing below the bottom of the blades with the rest of the slot above the blades. If you see a rectangular section then it’s probably fine. If you see an elongated section below the blades then it’s probably pulling too much fuel in from the transition circuit.

If it’s a vacuum secondary Holley, with it upside down see if you can open the secondary blade position until the transition slot is barely hidden by the throttle blade. There’s a little screw in a recess by the the throttle shaft on the underside of the base plate used to adjust the throttle plate position. It’s an interference fit so it takes a bit of effort to get it turning, but the slot is deep so it’s pretty hard to hurt it. If you can open the secondary blades up slightly, that allows you to close the primary blades some which will maintain your idle speed but will give you control of the A/F mixture and hopefully dieseling. If you still can’t close your throttle blades enough to get a rectangular slot profile, you may have to drill a small hole in your primary throttle blades on the front side to allow some air to pass for idle speed while closing down the blades to shut off the transition fuel flow. They usually start with 3/32“ to 1/8” to see if that works or if they need to be opened up further.

if you have a double pumper, mechanical secondary Holley then see if the primary blades can be opened slightly while keeping the transition slot covered and that will allow the primary blades to be closed slightly for a lower idle.