Have you ever replaced the o-ring on the throttle shaft ?
I had this give me problems trying to get the carbs synchronized, the idle jet, that's the one just inside the float bowl next to the idle mixture adjustment, the jet has two holes drilled in it at 90 degrees to each other .
There is a small orifice at the very end of the jet, mine was clogged, I got fooled when I sprayed carb cleaner into the jet at the screwdriver slot, it came out the holes in the side, nothing came out the very end .