The linkage usually goes to an air vane governor control under the sheet metal blower housing. Air from the flywheel moves the vane controlling throttle/rpm's.

Sometime the vane can get sticky but most often the hunting idle is a dirty or internally leaking fuel control issue in the carb. Check the vane for free movement, check for loose carb mountings causing a vacuum leak. If these fail pull the carb, clean it and put a kit in it with a new needle valve.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer