It's occurred to me that your issue could be an external problem with the governor. These engines have pneumatic governor that functions, somehow, using a choke like flap in the intake manifold which is connected via linkages to the IP. Thank Dog I've never had to do anything to it on my 115 but, if I ever do pull service on the governor, it should be entertaining. Somewhere here on the forum, there is a hilarious translation from German of the adjustment procedure. It starts with "Pull off at a shady place and open a beverage."

I will try to find the procedure in the FSM and somehow post it.

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Never a dull moment at Berry Hill Farm.