I have used a manometer (my old plumbing one)to balance my su's, to get it to work I had a piece of plastic that coved the ram tube then drilled one hole to fit the manometer tube into and then drilled a some small ones to let the carb get some air.

Place the plastic over the intake and adjust be drilling more holes or taping the holes up so the engine runs well and you can see the preasure reading.

I balanced it with my ears first then checked it with the manometer.