Popping through exhaust under partial throttle

[lvrpool32]

I have a "popping" in the passenger side sidepipe under partial throttle (Its not a missfire, the engine runs very smooth and the popping goes away under hard acceleration and the car pulls hard under acceleration). Now I am running a multi carb set up (4 del'orto's) and I know my vacuum flow is the same through all 8 venturi's.
I am thnking maybe a vacuum leak or maybe dirt in one of the carbs...anybody have ideas?


Nick