Either A your carb is too big for the cam and heads and your engine loses all airflow (vaccum) when you open the throttle completely too suddenly, or B its too lean to compensate for the sudden rush of air, you may have a bad, I believe its called a power valve (the thing with the arm that moves when you floor it to give it extra gas) check to make sure its moving when you rev it, maybe a bad diaphram inside. if the problem occurs in park you can hold that down yourself and rev it to see if it goes away. or C you could just simply be too rich and overloading the engine with gas too suddenly for it to deal with it.
all this of course assumes you carb doesnt need a rebuild or a fuel filter or something, and that you havent gotten the needles way out of wack trying to fix it.
edit: forgot the ignition stuff, make sure its all working right obviously, but like you mentioned retarded timing can cause that stumble because the engine literally has to catch up with the fuel and air when you hit the throttle.
so check it to see if it needs advancement.
power valve, that the term I couldnt remember, thanks pabear
