Sounds like its igniting before the intake valve shuts. Almost certainly the immediate cause is timing too advanced.
I'm all for measuring timing rpm relationship before opening up the dizzy. The springs effect the rest of the curve too, so I like to reduce the number of times I have to open up and modify. If your engine likes a quick curve, then filling in the inside end of the slot is the better way to get the initial. Just my preference. There's lots of ways to skin that cat.
Torque converter 'tightness' should have little impact at idle speeds and such low torque. The 'stall speed' goes down with less torque applied.
Power Valve has no impact on idle in most cases. The primary restriction for the fuel is the Idle Feed Restriction. It is usually much smaller than the main jets. When the area of the upstream restrictions are 4 times larger than the IFR it really doesn't matter if upstream flow is increased.
The 'square' transition slot is just an easy starting point. The best opening will vary a little depending on slot width, idle feed and air bleed restrictions, and engine.
Hysteric - Thanks for sharing your experience with that. Very interesting.
I agree with your explanation about the impact of exhaust gas dilution. In addition, my opening paragraph is true even for stock cams. In neutral, power to overcome internal friction, the alternator and the water pump is all that is needed. Lower power conditions allow for leaner mixes, which go hand in hand with more advance for the reason you explained. But it isn't enough power to do anything else. When I was just learning to tinker with cars, I had stock 6 cylinders and its the same deal (except I didn't understand why it would seem better with more and more advance in idle and leaned out, and then lousy when put in gear). Not 'til Shrinker explained it on the old Innovate forum did I really understand.
