Small Engines (Lawn Mowers, etc.): Kohler engine, reputable repair shop, lean condition

Question
I have a New Holland Lawn Mower with a 20 Horse CV20S Kohler.My problem started when the engine wouldn't start properly,it would rev up & then stall out,it was also making a rattleing noise which sounded like it was coming from the motor.I took it to a reputable repair shop & they said they cleaned the carb & set the rpm.The rattleing noise was coming from the muffler,they spot welded the end of it,apparently it the weld came loose.My problem now is that the engine just doesn't seem to have the power that it had before.When i mow uphill & mow in higher grass you can hear the rpm's going down & then when the mower comes to rest i can hear the rpm's going back up.It just seems like the governor isnt bringing it up to speed.I have had this mower for 5 years & i bought it new so i know it is not running right.I took it back to the shop & they said that they couldn't find anything wrong with it.Can you give me any suggestions.
Thanks
Bob

Answer
It sounds like it may be a lean condition that is making it produce less power.  If equipped, try opening the high speed mixture screw 1/8 of a turn.  If not equipped, you would need to look at a larger main jet (or drill the existing one open more with a precision drill).  The governor speed is determined at a no-load condition and the spring opens the throttle to WOT when the engine tries to drop in RPM (under load) so if the unloaded RPM is correct and the governor is responsive to load and unload, it is working correctly.  The only other issue could be that it is low on compression due to lots of hours.  Is it possible that a blade brake or the brake on the transmission is dragging?  Perhaps a bad idler pulley.  These are just further suggestions if the engine is working fine but the machine is low on power.  Hope this helps.