Small Engines (Lawn Mowers, etc.): garden tractor running funny, mtd yard machine, gas filter

Question
My MTD Yard Machine garden tractor (1998?) with a twin cylinder Briggs motor (20 HP Diamond I/C model # 460777) is surging. I have done a very basic tune up, oil, gas filter, cleaned & adjusted all linkages, air filter. It does not idle or run at full song without surging. Im stuck for the next step. It is a new tractor to me so Im not sure of the history.  

Answer
Hi John,

Is the surging an up and down "hunting" of the engine speed or a runaway type, to high speed?  A hunting condition is usually caused by a weak carb spring that works with the governor.  Also check for sticky linkage.  An uncontrolled high speed condition is caused by an air leak at the carb intake.

Let me know what you find.
Michael