rattling noise??

I have been searching online for some help with my 1985 Us Marine Force outboard. Model # 856x5l Serial # 2790. This outboard was on the 1986 Bayliner Trophy bass boat. We just purchased this boat last year. The gentleman that we purchased the boat from said that the last time he used it he took the boat to Canada fishing and the motor started to rattle. He shut it down right away and never started it again. He was going to be buying a bigger boat anyway. He thought that the bearings are out of it. The motor is not tight and it has compression. We are getting ready to work on it and was wondering if you had any ideas that might be helpful. I would appreciate anything that you could offer.

Re: rattling noise??

Well, you finally came to the correct place.

Before you do anything, get actual numbers for compression. If you are saying it has compression based on turning the flywheel by hand, be aware that the engine can have a bad cylinder and you will not be able to feel it. You MUST use a compression gauge.

Then, if all cylinders measure within 10-15 PSI of each other and over 100 PSI, try to BRIEFLY start the engine just to see if you can localize the noise.

The reason for this is that damaged bearings will sometimes sound like a knocking noise BUT a melted piston due to detonation will also sound like a knock when it initially happens.

If you should hear a knocking or tapping sound while cranking the engine for compression testing, then it is reasonable to suspect that a bearing set on one or more connecting rods has gone bad. HOWEVER: It is not unknown for these engines to egg shape the wrist pin holes in the piston. SO, if the compression is high and the knock is present either while cranking or running, remove the cylinder head and try to pull and push each piston. there should be no play.

If one or more cylinders show very low compression readings, then remove the cylinder head and examine all three cylinder bores. After you do this, come back with your observations.