High RPM on 21 HP B&S

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?

SRTsFZ6
So you only have 3,500-4000 operating RPM range?
 
Have you done any work or anything unusual happen to the machine recently?
 
Also, please post the model # of the engine.
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Yes, 3500 at lowest throttle position, 4000 on "run"
No recent work or anything unusual.......running fine until high RPM situation.  Occurred while mowing.
Craftsman Model 331877, Type 2371G5

Ok, here's a link to your engine:
https://www.partstree.com/models/331877-2371-g5-briggs-stratton-vertical-engine/ 
 
Watch this video, especially around the 11 minute mark and then check yours.
(He's kinda goofy but knows his stuff)
https://www.youtube.com/watch?v=XbFyGRixoGA 
 
 
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