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Well, I adjusted the trim tab this weekend; to the right, halfway between center and maximum allowed angle. It reduced the harmonics some. I keep the trim down and the vibration is less but the steering is hard to turn to the left. When I trim it up the steering loosens up but the vibration increases and the boat tends to porpoise. I do fiddle with the jackplate height also.I guess the next step is putting the trim tab maximum right and live with it. I just can't figure out the cause of the harmonic vibration short of taking the jackplate off. I don't know how to begin checking the fly wheel magnet. Kinda weird.