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My YTH 21 K 42 with a 21 hp Kohler 21 hp engine is surging when not under load .
When I engage the mower blades the surging stops . As soon as I disengage it will start surging again ..
Any thoughts on why this is happening

What octane gas are you using & which fuel treatment are you using?

Idle circuit in the carb needs cleaning, as this is a symptom of a lean running condition.

As Rivets said, dirty carb.

slomo

Can also be a slight play in the internal governor weights. On some I done the static governor adjustement by inserting a .010-.020 feeler gauge between the throttle vane shaft and its high speed stop which have cured this problem. Also make the minimum idle speed is set correctly.