How much Idle-Timing are you running?
Here's a possible scenario;
Too much IDLE-timing will run the IDLE-rpm up. Then
to get it down, you close the speed screw. Then
the engine runs out of fuel from the transfers, so next
you get more from the mixture screws, and now it idles.
But then, off-idle it is continuously too rich. But
worse is that when the throttle is closed at rpm, they close so far, that the vacuum in the intake just pulls huge fuel thru the mixture screws, which all puddles on the plenum-floor. And so, when you get back on it, she's gotta clean out all that fuel, and until it does, she is gonna run like she's flooded, cuz she is.
I mean IDK, but collapsing lifters is not on my radar.
Here's how I like to set the mixture screws.
I run the rpm up to cruise rpm, and lock it there. then I adjust the mixture screws for max rpm. then I kick the rpm back to idle.
If it won't idle there, it's probably wanting more fuel, so I increase the speed screw to get fuel from the transfers. If the speed gets too high, I retard the timing. After she's in the ballpark speed wise, she might need the mixture screws adjusted just a hair, but I try not to do that, cuz that is gonna go straight out the tailpipe at cruise rpm.
The above assumes that the fuel is fresh and at the correct WET level in the bowl.
Sometimes I adjust the fuel level as a fine-tuning measure.
For a 360, in a truck, I might target as much as 10* timing, to get the transfers synched to the preset mixture screws. If this makes the engine lazy at lower rpm, fix the timing curve!.
I have found a two-stage timing curve works very well. This requires TWO different springs in the distributor, one of which will have a long loop. Thus I can bring the timing in faster, to somewhere around stall speed or a little higher, and then slow it down to keep her out of detonation.
To find the ideal cruise timing I do this;
I run the rpm up to cruise rpm and keep it there for the duration of this test. Next, without regard to the timing light, I pull in the timing. If the rpm goes up, I drop it back down to cruise rpm and repeat, until more timing fails to produce more rpm at the reference cruise-rpm.
Next I put the timing light on her, buz her back up to cruise rpm, read it, subtract 3* for load compensation, and drop the rpm back to idle.
Now you have THREE data points, namely, Idle timing from before, and Power-timing is always ~35* for a 360, and now, cruise timing. Now all you have to do is marry them inside your distributor.
BTW, your Vcan can can be modified up to at least 22*, by grinding off the stops on the arm, until the diaphragm inside bottoms out. and the rate of advance coming in, can be adjusted by accessing the allen screw adjustment, inside, thru the vacuum nipple.
This is what I do.
FWI:
Your engine does not much care about idle timing. It's never gonna be ideal no matter what you set it to. Well yeah you can make it ideal, but, you just can't drive it there.
The first time it really cares is at stall-rpm; and
the second time is WOT, somewhere around 3400 +/- 200rpm, and
the third time is while cruising. which could be as much as 56* depending on load and rpm.
IDK if any of this is applicable to your situation, but if you are running say 18* of Idle-Timing, then I bet it is. As a test, just back it down to say 8*, and return the Idle-Speed to 700 in neutral, and see what happens.
