Yanmar 3YM30 wont rev and dumping black fuel

[wynmead]

Recently motor decided not to rev while underway above about 2400RPM. (max aaround 3600) Back at the dock reved to max no problem (out of gear).
Noted today same problem and if put on more throttle no extra revs (over 2400) but gobs of black stuff coming out the exhaust. As long as I did not increase the throttle beyond what it would be for around 2400 no problem. (In neutral reved to 3600)
Any more throttle and gobs etc. Motor sounded perfect throughout. No overheating, perfect oil pressure.
Ideas for investigation?
Motor would have around 400 hours on it.
Thanks

[jimbunyard]

Fouled prop

[wynmead]

No. Was acting up before I just did the annual haul out. Kiwi prop all good.

[jimbunyard]

Well if it still revs up to 3600 out of gear and only 2400 in gear, that certainly seems to point to a drive line problem. At least that's where I'd start. How far will it rev in reverse?

[DeepFrz]

I wonder if the Kiwi prop got adjusted during the cleaning process?

[djmarchand]

Most likely a plugged up exhaust elbow. This was a well known problem on the YMs predecessor, the GM engine. So the same thing may be happening. 400 hours is low. Usually it didn't start until 1,000 hours on the GM.

The plugged elbow causes a huge back pressure that doesn't allow enough air to be sucked in on the intake stroke. The engine is starved for air and so it smokes bad.

Pull the exhaust elbow off and look inside. Sometimes you can poke a screw driver into the raw water injection nozzle and feel scale and soot built up.

You can't really clean the elbow so replace it.

David

[DennisDW]

Dave's spot on. Exhaust elbow partially clogged most likely

My 3GM 30 became progressively harder to start as an initial symptom. Then it wouldn't rev under load and smoked. The elbow was clogged at the injection point and only about a quarter sized opening remained.

As a note, the elbow had a left hand thread on the intake side so that with a special pipe off the manifold, the elbow could be removed by just turning the pipe, not spinning the whole fitting. Too clever by half as it fooled me.

Good luck,
Dennis

[wynmead]

Thanks for the replies. Pretty certain the prop is ok. Greased the kiwi prop blades but not readjusted and exactly the same thing was happening just before it was serviced. On reading other threads the gunk coming out does look like batches of soot.

[Graeme Colmer]

I recently had a very similar issue with a 3ym30 gradually lost revs pumped out black soot about 400 hours
Removed the u bend from rear of heat exchanger found it almost fully blocked with sheet type scale / rust
I chipped it off opened up the channel put it back in and better than ever since new
Fantastic. However I removed so much metal scale that I was concerned pretty soon saltwater might be able to back fill hence bugger engine
I bought a new bend dipped it in Epinamel PR250 an Epoxy two part used on raw steel reinforcing for below salt water etc
I let it cure, painted it with a high temp silver spray paint and put it back it
I'll inspect it next year but I think it just might last 8-10 years no problem. The last one lasted 6 years however I had never checked it or cleaned out
Motor now runs better than ever no smoke full revs3600 under full load
Elbow in Australia not cheap at $220 but cast iron has no protection at all. The temp range required is low having checked it with an infrared pyrometers

I hope this helps you because I had some angst when looking at the same problem with little diesel knowledge.

[wynmead]

Thanks GC. I will be getting the elbow inspected and I suspect replaced. Just to add to the delights of owning a Yanmar the starter solenoid is also playing up. On looking at threads seems sub optimal wiring and connections to solenoid a common theme on the 3YM30! The black hole is all consuming,, but fun.. cheers