My Cub Cadet engine used to run very smoothly. Lately, it modulates when the engine is not under load. The rpm's increase and decrease every second or two. When under load, it runs great. It has been maintained according to manufacturer's specs (I'm seeing the exact same behavior in my Craftsman Garden tractor) What's wrong (MDF made machines)?
What you are seeing is the classic symptom of a partially clogged idle circuit in the carb. they carb need to be removed and given a good cleaning, and hopefully that will fix it.
