YT3000 engine surges

[DFoote]

I own a Craftsman YT3000
riding mower with a 21 hp Briggs Platinum engine that I bought new almost 10 years ago. The engine rpm surges and has for years. The carb was recently replaced (after I let it sit to long with ethanol fuel) but the surging still remains. Any ideas on how to correct this?

 

[bertsmobile1]

Surging is usually ( 99,9% ) caused by insuficient fuel to maintain a constant speed so the governor is always adjusting the speed .
Even new carbs can surge, particular if it was an online cheapie from Ammo zone or Evil pay as they are the prime markets for defective parts bought as scrap metal then sold as " fits a " or " OEM For "
After that you have air leaks between the carb & the intake valve
Way down the list is a leaking head gasket
and right own the bottom is a faulty governor

Start the engine, set the parking brake then get off & work the governor arm with your finger
Try to hold it at a constant high speed ( note I mean normal running speed )
If you can hold it and the engine does not miss this points to a failing governor.
Check for air leaks by saturating the carb & manifold with WD 40 or similar from a squirt / trigger pack ( not a pressure can )
Work the governor arm back & forward slowly to get a good vaccuum
If you get smoke from the exhaust you have an air leak .