Small Engines (Lawn Mowers, etc.): sears intek 25hp, mixture adjustments, throttle shaft

Question
I have a sears lawn tractor with the 25hp intek v twin. the engine surges, especially at idle.
engine has about 100 hours on it. I had this problem in the past when i still had the warranty on the tractor and the sears guy said - oh yeah that's the ...... (and i forgot what that was).
all the obvious suspects have been checked - fuel filter, air filter, fuel pump, etc.
There are no standard idle/mixture adjustments on this engine and I think it has some sort of solenoid.
Thanks for your help.

Answer
Hiya Steve
Most likely suspect is the govenor spring.  It attaches to the Govenor arm and connects it to the throttle linkage.  Easy for a shop to diagnose and replace, difficult for the do it yourselfer to replace (the springs are delicate and ackward to get to).  I recommend letting the shop do it, and ask for a warranty on the repair.

Next suspect would be an air leak aka- bad gasket or worn throttle shaft seal in the carb body.  These are more expensive repairs and definately a shop item.  Lemme know how it goes !