B&S 3.5hp Quantum won't slow down?

My 3.5hp Briggs & Stratton starts fine, but I can't get it to run below maximum revs.

The controls on top of the carb all move, but I can't make sense of how they are supposed to link up, does anyone know of a diagram/drawing for these please?

With very careful movement of the throttle I can get the revs to drop a little, but this is accompanied by what I understand as "hunting" and backfiring through the carb.

Is this connected with the throttle linkages on top of the carb? As I said these are connected, but don't seem to operate smoothly.

Any ideas would be welcome!

I have also (mistakenly) added this in the answer column to a previous question - sorry!