Oil in one cylinder

[kevmacmills]

I have a 1989 Sundancer 268 and I put new plugs in last year. On the last trip of last year, my engine was running rough. It started out with me wondering if the timing was off, but it got progressively worse to the point where it was missing, shuddering and generally unhappy. I pulled the plugs when I got home and they all looked normal... except for cylinder #3 - completely fouled with oil. I changed the #3 plug and put it away for the winter.

We got a late start this year, but I had it out for the first time yesterday and immediately noticed the same rough running. It was hesitating, surging and I was getting more vibration than normal. I decided not to stress things and stayed off plane and putted around. Got home and pulled the plugs out of cylinder #3 and #5 (the adjacent plug). #5 looked normal and #3 was oily (see attached picture)

My googling says it could be the valve stem seal or maybe the rings. What do you guys thing I'm dealing with here? How does one diagnose this?

Thanks for any help!

 

[northern]

If it is rings they could be carboned up. A possible fix is to do what a Tipple A Dragster mechanic did to my last boat. He took 15 OZ of tap water and poured it into the carb very slowly with the engine RPM at 1500. His comment, it would clean the carbon out or blow up the engine. It cleaned the carbon out and all cylinders had same compression before one cylinder was 90PSI. As to valve there is no simple fix.

 

[Maggieiscrazy]

You need to start by doing a compression test. If that cylinder has significantly less compression than the other cylinders you know you either have worn or failing rings, issues with valves or possibly a cracked piston. When doing a compression test you may find that your having issues with other cylinders as well.

 

[hottoddie]

Does it run smooth with a fresh plug in #3? If it does I would bet on valve stem seals. Is the engine original to the boat? How many hours? Does it smoke a little on start up or when running at high rpm. Valve stem seals will cause smoke on start up and rings will at higher rpm after warm up.

 

[kevmacmills]

Does it run smooth with a fresh plug in #3? If it does I would bet on valve stem seals. Is the engine original to the boat? How many hours? Does it smoke a little on start up or when running at high rpm. Valve stem seals will cause smoke on start up and rings will at higher rpm after warm up.

On the fresh plug, it ran OK for a bit but I noticed the rough running within 20 minutes and things got worse during the day. I haven't noticed any smoke. As for hours, I'm guessing somewhere around 800. The hour meter was broken when I bought the boat and it was reading 590. I replaced the meter and have put about 150 hours on it.

 

[Maggieiscrazy]

Are you sure it’s oil on the plug and not gas? If your not noticing any smoke it doesn’t sound like it would be oil. It sounds more like the plug is fouling out with gas.

 

[scoflaw]

Compression check

 

[hottoddie]

I would try a new plug wire on #3 to see if the plug continues to foul. Of course a compression test is always a good place to begin.