493 carburetor tuning question

Could be the wires to the amplifier are reversed. Assuming this isn't an RTR model.
1/4" of engagement of the tab in the slot should be enough. Doesn't sound like you've got a B in an RB block. I
'll leave that for the guys who have built B/RB engines.

Up down motion of the distributor drive shaft should be zero.
The intermediate gear moving up down can cause an apparent shift in timing because of the helical gear. 12* seems like a lot for that. 3-4* OK maybe.
Same with the distributor tab into the slot of the gear. If you place the distributor in the engine, can you move the rotor 6 degrees?