I just took my 200 h.p. Mercury 6-cylinder (no EFI) out for the first time this year since having my dealer winterize it last October. The motor fired right up and ran great in idle. After I cleared the harbor and hit it full throttle, a noticeably loud rattling / knocking noise came from the engine above 3000 rpm that sounded like a bolt was banging around in the flywheel housing. I thought that maybe a spark plug wire was loose from the winterizing, but everything looks good. The motor was running great when I had it winterized, now it's making this odd noise but still runs seemingly well. It seems like the knocking lessened after I had run about 20 gallons of fuel out (I know, I was stupid to keep running it), but maybe I'm just wishfully thinking that I won't have to spend $3,000 for an engine repair! My dealer says it sounds like something loose inside the motor (bad bearing maybe) and wants to tear the engine apart to inspect it. I am thinking it might just be bad fuel if they didn't stabiilize it correctly. I pre-mix my fuel in the tank since my oil injector was removed by the previous owner years ago. Should I try fresh fuel before I let the mechanic tear my motor apart? As a side note, I had added a fuel flowmeter to the fuel line over the winter, would a lack of fuel flow be causing this knocking sound? I've also noticed that when in idle, my fuel line slowly drains back to the tank and kills the motor after a few minutes, almost like I have a loss of suction from the motor. I've replaced all fuel lines, bulbs, filter etc. up to the motor but haven't replaced anything in the motor yet. Again, would a fuel loss issue be causing the knocking as well? Grasping for straws now, but I trust someone here can set me straight and tell me the really bad news. Thanks!
