B & S Running at High RPM

I have a 21HP B&S on a Craftsman rider. During the last mow the engine began operating at a much higher RPM in the "run" throttle position . It was not surging. Engine rpm would reduce when throttle backed off. Check of RPM in run position showed 4000 rpm. It would run at about 3500 at low throttle.  Governor assembly appears to be OK and intact and operating at various throttle positions. Any ideas?
 
 
 
 
 
Reply