I've got a four winns 20' sundowner with the 5.0 (ford) omc engine. Here is my scenario; I bought the boat which was sitting for a year. I drained the fuel tank, change the filters, premium gas with injector cleaner, checked all fluids, changed the plugs, checked the timing, and took it out to run it. It runs decently with a smooth idle and enough power to get up on plane. The only issue I'm having is a strange non-consistant knocking noise. It only seems to start around 2,5k rpm and is constant above 4.2k rpm. What's odd is that it doesn't seem to affect performance or cause the engine to stumble. It is irratic and sounds like it is coming from the rear of the engine compartment. The boat will still go 30-35 mph so I'm not thinking it is fuel related, but it might be. I tested the pressure when it was started and all fell into spec. I tried running it higher rpm's in nuetral with the muffs and couldn't hear it pop. I thought maybe lower unit or connections but I know there isn't any load on the engine in nuetral. The sound is like a matal rapping on another peice of metal; almost like a popping noise. I was thinking maybe the u-joints? Maybe some other know issue with the engine? When I changed the plugs all looked great except for one that was also a different plug so I think they couldn't get it out before when they changed them and left the one in. I noticed that one exhaust manifold was slightly hotter than the other but it doesn't nessesarily mean anything. Any advice or what I should check?
