I am getting a pop coming from the carb whenever I hit it with a blast of fuel. I can roll on slow and it doesnt pop. It also pops, but more like .22 caliber gun shots. Whimpy so to speak. Looking into the carb with without air cleaner on, looks like every time it sprays a nice chunk of from the nozzle, I get a pop coming back out from the throat. I have a shim on a stock needle. 48/180, 1.75 turns on air screw. cv carb. Evo

Would losing the shim lean it up enough to stop the pops? Im guessing I'm off the pilot and on the needle when its happening but I dont always see the piston raise the needle any and I still get the pops if nozzle sprays

 

when you yank the throttle, the engine will go lean, the reason for the accel pump. also when the throttle is yanked, the velocity will drop to near atmospheric so less fuel is drawn up through the jets.
make sure the nozzle is in correct position and the hole is not restricted and it is consistent with volume. also, there is a hole in the bowl that supplies fuel to the chamber that must be clear and the check ball is sealing. also the rubber is in good shape and the timing is correct.
as far as piston movement, it will be far less on an unloaded engine, aka testing like you said.
any changes to the exh side??? that to can effect intake flow.
how about engine balance, both cyl pulling the same.