I thought I had the same issue with my Merc 25, I adjusted the tab both directions and everywhere in between and nothing helped. Found out through trial and error and a little dumb luck my bushings/pads that go between the steering hinge clamp and the shaft were worn out. Replaced them and all was well.
Hi – bumping up an old post but I am having the exact same issue with my Merc 25. Tried adjusting the trim tab both ways but it still pulls hard right. Can you PM me the details on this fix with the bushings? Thanks!