F225 Knocking - Help Please!!

[mjc83930]

Hello,

I recently purchased a 2004 boat with a Yamaha 225 Outboard (600 hours supposedly). The motor has been running flawlessly for the last few months and after I returned from a trip yesterday the motor starting knocking fairly loudly. The knocking happens at all RPM's and idle. It is coming directly from top left side of the valve cover. In doing some research I learned it could be the Timing Chain Tensioner. The mechanic pulled the valve covers and inspected as much as he could (valve clearance, piston head, tensioner, compression, spark plugs, etc.) and advised that everything was well within spec and looked good. He didn't feel like it was the timing chain tensioner, as it did not look worn and the timing chain appeared to have no play, but to me it sounds like it is coming directly from where that part is located. He now wants to tear it down further to see if it is possibly the wrist pin of the piston or crankshaft related. Before I sink thousands into this diagnosis I was hoping that someone could help further isolate the problem or offer suggestions. I posted a youtube video of the knocking sound it is making (see below). Sorry that the video is so poor....

https://youtu.be/a7RJI28Rl0o

Thanks for any help you can provide!!
Marty

[TownsendsFJR1300]

Wish you revved it a little bit, maybe to 1500 rpm's...

Anyway, if the top end is tight, it could very well be a rod bearing but that's usually more of a double knock(as the piston changes directions up and down)

Couple things you can check / do.

One at a time, pull off a spark plug wire, (ground it) and start the engine. Do those for all cylinders suspected and see if the noise lessens. No spark, no firing in the bad cylinder may make the knock less..

Next, pull the spark plugs and by hand rotate the engine noting each piston as it rises and then starts to go DOWN. Put a CLEAN screwdriver down the hole and push paying REAL HARD ATTENTION. If you can feel the piston move(it will be very LITTLE MOVEMENT (and should hear a slight clunk as the piston pushes back down against the crank. The rod to crank bearing likely toast..

Also, drain the oil into SUPER CLEAN PAN and drain the oil filter as well into the pan. You want to look for any metal debris. Any is NOT good. You can mail out a small sample of the oil to companies that check the oil. They can find excess bearing material in the oil if any(IE a toasted bearing) JUST by doing an analysis on it...If bearing material is found, you can make a better decision, rebuild, new power head, etc.

You didn't mention it, but is the oil level normal, smell like gas or WAY over the correct level?

[fairdeal]

I had a sort of hollow "knock" sound develop on my F225 -
turned out to be the port side cam gear.

But it was only after the belt was removed that it was apparent where the sound originated. I think the valve cover acted as a "resonator".

https://vid.me/tKUi

[TownsendsFJR1300]

Very possible too, ^^^. that "sprocket" is aluminum tightened down to a steel cam shaft. With time wear, the aluminum literally squishes down allowing that play.

Regular Harleys use the an aluminum drive "sprocket" for the final drive of their bikes with a belt that is super tight. Between the tension and the two different materials, it can loosen up.

My friend had a HD Sportster and his sprocket, (up in the high mountains of Tennessee on vacation) failed. The steel transmission shaft (or cam in your case) literally stripped out the grooves in the "sprocket". Lots of noise, no go...