Yanmar 2QM15 Cylinder Woes - New Boat Owner

[jrsd]

Hi Everyone,

I realize I've never posted on this forum before and I'm asking a potentially complicated question, but I'm hoping anyone can give me some insight into the problem I'm experiencing with my Yanmar 2QM15 (in my 1980 Hunter 33).

My problem:

Only one of the two cylinders is firing, and the boat is pumping out black smoke and particulate matter into the water.

Background:

For the past few weeks, especially running at higher RPMs, I noticed that more exhaust than normal was coming out of the boat. It wasn't a lot, so I didn't make much of it. When I was bringing my boat in from a trip, I noticed that the RPMs changed slightly with the throttle in the same position. Since I'm new to boating and engines, I wanted to start doing regular maintenance on the engine. Since I had the parts, I started with the secondary fuel filter and the air intake filter.

Not understanding that one has to bleed the lines after replacing the secondary fuel filter, I cranked the engine 5 or 6 times, and then got help since it wasn't starting. A friend of mine came down and bled the lines, but when the engine fired up it started dumping large quantities of the particulate matter in the water and billowed out black smoke for about a minute. The water started clearing up and the black exhaust disappeared, however, when I attempted to put the boat into gear, the RPMs dropped dramatically and the engine started putting out black exhaust as if it was under a heavy load.

Investigation:

A mechanic was pretty certain that the engine was being suffocated by the elbow exhaust mixer - which he showed me was about 1/4-1/3 clogged. But yesterday, a friend of mine took a look at the engine and we found that it was firing on only one cylinder. We checked to make sure it was getting fuel, which it was. So I decided to pull the injector and have it tested, and it came back fine today. When I was pulling the injector, I noticed that one of the fastening nuts was a little lose, but I didn't think that would be enough to have the whole cylinder lose compression - and nothing was leaking out and it wasn't making any noises whatsoever.

Next Steps:

Firstly, what are some of the possible problems that could be happening here? Secondly, what would be my next steps to further troubleshoot the problem?

Notes:

- The boat is dived regularly, and I verified that the through holes and prop were clear.
- I've done basic maintenance since I purchased the boat, like oil and oil-filter changes, added a new seawater strainer, etc. I've put 25 hours on the engine since then.

Thank you very much for your time and insights,

Robert

[lars]

i think the exhaust elbow is certainly part of your problem, if it's origininal it's way overdue for replacement anyway. check for air in the fuel system and adjust the valves, all these are standard maintenance on these little engines