Engine knocking bad after repitching prop

[texantommyray]

I have a 1987/88 Yamaha 90 and had it on a 16ft bow rider. I took it off and mounted it on a 20ft pontoon. It ran great. I have always replaced the lower unit fluid and propeller every year. I start it up every week or so.
It was running great but i noticed the RPM's at WOT were 3800. The prop was 13 1/2 x 19pitch. So I bought a 14x13 and the RPM's went up t0 4800. I called the mechanic and he said bring the prop back and he would replace it with a 14x11pitch. So I did. Then it ran 5600 RPM's at WOT. I was told that is where my motor needed to be. So I ran it WOT for about 20 minutes to get to the other side of the lake in slightly rough water and when I started backing throttle down I heard a loud knock/clatter sound. It was still running but knocking pretty bad. I drove it at around 1800 RPMS at a really low speed just to get back in to my slip. I tested compression and found (top to bottom) 120, 85, 120. I took head cover off and when I turn flywheel they all move up and down freely and all look good, except the middle piston doesnt come all the way up. It stops at aboutb 1/4 inch or so from the top.
Piston doesnt seem to be loose from this side.

I probably should have left the prop alone. It was running fined for a month before this happened.

does anyone have any ideas of what this could be?? Loose rod maybe??

Thanks
Tommy

 

[Dhadley]

Re: Engine knocking bad after repitching prop

First guess would be a damaged rod bearing or wrist pin bearing. While the overpropping may not have caused it completely, it sure didn't help the situation. The other things pontoons are famous for is not allowing the prop to stay hooked up all the time. It'll grab, slip, grab etc. That's really hard on wrist pin bearings. There are pontoon specific props to help that issue.

 

[steelespike]

Re: Engine knocking bad after repitching prop

If that is an inline motor I believe it uses 3 carbs.If one carb was lean that might cause a bearing failure.

 

[john from md]

Re: Engine knocking bad after repitching prop

The engine was telling you something but you were not listening. The needle bearings in the connecting rod were starting to go, maybe at both ends. This led to the lower rpm that you were seeing. Overproping just sped up the friction and bearing disintegration. The reason the piston won't come up to the top is that the bearings have probably disentigrated.

I did this once in the ocean off of Wildwood New Jersey. I kept it running at about 10 mph all the way back to the dock. Two needle bearings and one main bearing had started to disassemble. And in those days, there was no such thing as Boat Tow!!!


Considering the age of the engine, I would do an overhaul rather than fix the one cylinder. If you don't, you will be doing it again in the near future.

John

 

[texantommyray]

Re: Engine knocking bad after repitching prop

Thanks everyone for the help! Im going to take it apart this week and let ya'll know what I find. Is this one of those $2000-3000 jobs im looking at?

 

[steelespike]

Re: Engine knocking bad after repitching prop

Yup, almost 20 years ago they replace one piston in a 88 50 Evinrude no bearing problems.Quality 50+year evinrude dealer $800.Its still running today.

 

[texantommyray]

Re: Engine knocking bad after repitching prop

I could live with 800.00. Hopefully thats all it is.