Small Engines (Lawn Mowers, etc.): Murray riding lawn mower, 12 Hp Briggs, surging, hp briggs, speed rabbit

Question
My Briggs engine "surges" as I ride the mower in slow speeds; this causes me to lurch forward and back in my seat.  At the highest speed (rabbit), I can cut my grass with no "surging," but it is dangerous to do this.  How can I correct this with some adjustment to the carburetor and/or governor?  I've put in new gas, new gas filter, new air cleaner, new muffler, and new fuel line.
Many thanks.    Roger

Answer
Actually, it is best to cut the grass at full rpm, with the trans in a low gear, the engine and deck are designed to mow at @ 3200-3400
rpm.

There may be a low speed mixture screw on the side of the carb,
which can be turned left to richen up the mix and get rid of the
surging.  If you can find the engine's model numbers, I can tell you for sure.

Thanks,

Fish