Im about to take my boat out of the water and have the same problem with knocking. As well lots of black soot all over the back of the boat were the exaust comes out. If I bring the rpm up about 1200 rpm it will seam to get better. I was going to get the injector rebuilt ($60.00 canadian) and adjust the valves over the winter. New fuel filter last season but I will cange them out aswell. I hope the injector solves this problem.
Any thing else????