My guess is that your
Engine went rich from getting too much fuel, that there is now liquid fuel just laying on the intake floor, and throttle is Too Far Closed to deal with the migrating/evaporating fuel.
IMO; here are your options; do as many as it takes to solve the problem.
First;
1) make sure the choke is fully off and stays off, and
2) if you have the stock factory cast iron intake, make sure the carb-heating Crossover is working, and make sure the throttle return spring is not something off a Mack truck! then
3) make sure the Wet fuel level is as specified by the manufacturer of the carb.
4) make sure the metering rods are staying down at idle, and are reluctant to jump up when cracking the throttle in Park/Neutral, with no load on the crank.
If yours jump up instantly one of three things is wrong; A) the springs under the pistons are too powerful or B) there is a load on the crank, or C) the exhaust is restricted. See below;
5) Increase the curb idle screw at least one full turn.
6) Decrease the Mixture screw adjustment 1/4 turn each, and
7) Then retard the timing to whatever it takes to idle at not more than 800, or down to 5 degrees Idle-timing, whichever comes first.
If you retard the Idle-Timing, then you may have to recurve your distributor.
DO NOT, let anybody preach to you that you need 18 degrees of idle-timing with a stock low-stall convertor. That's just looking for trouble.
8) Decrease the accelerator pump shot
9) open the plug gap to .045, and get a coil that can consistently fire that. I suggest the Big Yellow Square-top Accel SuperCoil.
10) If you have log-manifolds and a single exhaust, criminy, put the 2-barrel back on.
Hints;
>If you suspect there is a load on the crank;
get the rear wheels off the ground, and slap the throttle. If the problem goes away, Badaboom!
>If you suspect a restricted exhaust;
plumb a low-pressure gauge into the pipe just ahead of the muffler, then take the car for a road test. No matter how hard you try, in at least two gears, the pressure should not exceed 4psi. If it does, replumb your gauge to the rear side of the muffler. If the pressure is considerably less there, than what you measured in the front of it, cut the muffler out and retry.
>The big deal here is that the engine went rich at the throttle slap, and the throttle closed too far and/or too fast to deal with emptying out the intake, after the slap.
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Lemmee tell you something;
If you bring your Duster to me, and I find the following: that
your cam-timing is correct or close to it, and
the cam is a 292/292/108 or less (the biggest I have street-tuned), and
the CCP is even, more than 130psi, and the Leakdown is less than 4%, and
that you are operating between sea-level and 1000ft , and
the engine has no vacuum leaks,
then,
I can guarantee you that,
I can tune your engine to idle just fine, very possibly/probably with as little as 5 degrees of IDLE TIMING, and I will be able to set the idle-rpm down to 550 in gear.
This is NOT A BRAG. This is just a plain fact.
Anybody, and I mean, anybody with a reasonable amount of skill and experience, should be able to do the same. Running too rich after a quick deceleration makes sense to me. I will try out some of your ideas. Thanks for taking the time to write all that! Also I have an eddlebrock aluminum intake and also duel exhaust with thin walled racing mufflers(which I need to change as they are loud as hell) the guy I bought the car from owned a muffler shop and installed those. I did notice also there is some homemade stuff going on with the throttle assembly, springs and brackets. I’ll post a pic
