How much RPM drop from park to drive with stock type 340 convertor
The drop in rpm at idle has only a small relationship to the change in converter.
I'll bet the drop at idle is mostly due to the engine and the idle tune.
Mattax nails it!
The idle drop is caused by one major thing; and maybe two other minor things.
1) transfer slot sync, and
2) transfer slot sync, and
3) probably you guessed it, the transfer slot sync.
What I see most of, is guys throwing too much idle timing at the engine, which then causes the slots to be too short, which leans out the idle, requiring major compensation from the mixture screws. And BadaBoom she idles.
But then you put her into gear, and just like that, the airspeed slows and the fuel stops flowing, and she stalls, or near-stalls.
Your engine does not need 20* of idle timing.
It doesn't even need 18 or even 16.
It needs whatever it takes to get the transfer lot working right.
Lest you doubt this;
my manual trans 367 will pull itself around, on level flat hard pavement, at 550 rpm, @5* idle-timing, with a 230*@.050 cam in first gear with a 10.97 starter gear. It also did that with a 292/292/108 cam, but at 600rpm with a 9.44 starter gear.
The point is, let the engine tell you what she wants for timing by first establishing the transfer slot exposure under the primary throttle blades. Then don't be in a hurry to crank up the advance.
For your combo, if no one has mentioned it, I would set the transfer-slot starting point, to square; that is to say, just as high as wide, and put the mixture screws to the center of their adjustment range, about 2.0 turns on an AVS-type carb. After this; DO NOT Touch the speed screw! Set the idle-speed with timing. Do not look at the number; just push/pull the Vcan until you get 600 in gear.
That will be your starting point.
Now put it into Park/Neutral and see what happens.
Then go back to post #15
