Engine problem

I was with my dad in his boat this past week. We encountered an
engine problem while out on the boat. He has a 115 hp mercury engine.
It's a 1979 engine I believe, but has been well taken care of, evidence by
the fact that we haven't encountered many problems before. Anyway, we
had the boat on full plane running wide open when it suddenly just failed.
The engine just kinda made a noise and stopped running. After we got
towed to dock we tried turning it over, but the flywheel would turn real slow and rough sounding, but no hint of actually starting. We tried each of the next couple days and no change. Does anyone have any idea what this sounds like the problem might be. Any help appreciated. Thanks.

Re: Engine problem

You can start off by checking all battery connections. They must be clean and tight. Make sure the battery is fully charged. Do a compression check. Check for a stong spark at all plugs. Let us know what you find.

Re: Engine problem

A couple of ideas come to mind
1. Bad impellar or picked up a plastic bag stopping cooling water and overheated the engine.
2. No oil in fuel or bad oil pump and burnt up engine.

Take plugs out and inspect. You might see them covered in aluminum from burnt up pistons.
Do a compression test.

Re: Engine problem

Just a guess, but maybe plugged carb caused lean mixture on one cylinder, fried that piston. Start by pulling spark plugs, check for burned looking plugs, then pull heads. Motor may be toast. Good Luck!

Re: Engine problem

Check the lower unit oil 1st as the gearcase may have went south and locked engine down. If the oil is burnt,grey with metal shavings or stinky water drop unit and see if it frees up. If not it has a piston problem as suggested from earlier posts....

Re: Engine problem

We took it to our local mechanic close to here that we know. He did a compression test and the third cylinder was getting no reading. He seems to think the same as most of you...likely a fried piston. Thanks for all your suggestions. Considering the age of the boat, we likely will look into getting a newer used motor if that makes sense. The one on there is 30 years old, probably finally failed. Any suggestions on affordable used motors. Doesn't make alot of sense to put a nice new motor on a 30 year old boat. Thanks.