I would say you are on the transfer slots and not the idle circut pulling advanc in on the dist at 1000 rpm along with that big drop in rpms when going into gear.
it should only drop 100 rpm or so when going into gear.
have you put a vac gauge on the carb?
is it steady vac or jumping around like the damper/timing mark?
I would get a lite on it along side a vac gauge and adjust timeing with the gauge then adjust carb idle air screws.
I use the lite to make notes on the timeing setings,I set the dist by vac gauge and highest steady vac reading,then adjust air screws.
make a few runs at different settings to see what works best.
last one i could not tune the miss out of was a cracked head leaking water into a cyl.
I would still pull the plugs again and recheck for a fouled plug. i would replace them just to make sure.
look it over at night to see if it is jumping fire on plug wires and run you hand down each wire while it idles.
