Small Engines (Lawn Mowers, etc.): Briggs 12 HP, Synchro-Balanced engine -- surging issue, murray riding mower, hp briggs

Question
Hello, Timothy:
I have a "surging" issue with a 12 HP Briggs engine on a 1990 Murray riding mower.  When I ride the mower out of my garage, the engine "surges" back and forth causing my body to lunge back and forth in the seat.  The Model and Type number is:

     281707 0415 01 90081431

I've installed all new spark plug, air filter, fuel filter, fuel line, and fresh gas.  Can I adjust the fuel mix or do I need to mess with the governor?   Many thanks for your help.   Sincerely,  Roger

Answer
Hello Roger,we have two things too think about. One we have a governor:the primary function of a governor is to control the engine at the desired speed. Two the more load is applied, the more the governor will open the throttle, allowing more fuel and air mixture.As the load increases, the governor increases the fuel mixture and, as the load decreases, the governor will cut back on the fuel mixture. This reduces the horsepower output but still maintains the same constant speed. Moreover, find out what kind of governor you have on that; could it be a air vane or centrifugal? Get yourself a manual. I would also make this job a two person job reason being is first make adjuustments then work it from there. Could also be that you have a break in the gasket.