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When they changed the lower unit gear ratio (2015), they had to change the pitch of the prop. It's a math thing. Not related to your issues.

A prop has a large insert that is pressed into the prop that has the correct splines for the engine it is installed on. it also has a shock absorbing material to protect the gear case.

Due to age or hitting an object, the hub can slip at medium to high speeds. It often does not slip at lower speeds.

Do not use any of the original gas. Eliminate that as an issue. Run on fresh fuel.

Given your low rpm's, on reflection, I think the engine is not running on all cylinders. Test each one for spark.