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Have you checked the fluid level in your trim pump? Not sure where it is on your model of Yamaha but a service manual will tell you. If the previous owner replaced the trim pump, that would be the first thing I'd check. Next I would go through the hydraulic lines to make sure those are routed correctly and there's no crimps that would restrict hydraulic fluid flow from the pump to the hydraulic trim ram. Lastly, if that fin makes your trim too sensitive, try removing it and seeing how the boat responds. Boats shouldn't need those silly things.