Thought I would give this a shot on this board, bunch of clever people with engine experience...

I sold a 1983 Mirage 25 sailboat with a Yanmar 1GM10 (single cylinder 10hp) that was freshly rebuilt. Conventional shaft/strut installation. Engine worked fine when I had it. My neighbor (the buyer) is now having problems.

Not loaded, engine will rev up to 3600 rpm and stay there. Under load (propulsion) the engine will only go up to about 2100 rpm, then after two minutes or so, bog down and slow, then die, blowing a bunch of black smoke. Boat is far from hull speed.

25 foot sailboat, fin keel, 4400 pounds, 4ft draft, 10 foot beam. It had a 14x14 Martec folding prop when I had it (and would hit hull speed at about 3000 rpm) and worked great. He removed this prop and went down to a 13x9 folding prop, and now is going to go to even a smaller prop to solve what he thought was the boat having too big a prop. I think the prop was not/is not the issue. He now has an old 6hp outboard on the boat, and it pushes the boat faster than the misbehaiving 10 hp diesel.

A certified Yanmar repair shop rebuilt the injector pump, and it was installed and timed. A new fuel injector was put on. Engine has good compression, and compression release is not sticking. Throttle linkage is working fine. Stuffing box is not too tight. Shaft is not bent. Plenty of fuel getting to engine. Replaced fuel filters. Starts fine, runs fine no load. It has a governor, but with a propulsion engine in flat water, the governor doesn't really do much of anything (although it is working properly).

Any thoughts?

Matt

I had a similar problem on my 38' gaff rigged sloop and it turned out to be a crab pot wound up on the prop. Just a thought. I had to go under it in November and cut the pot off. Nearly froze.

I don't know if I could say that the symptoms you describe point here, but I had a similar engine (3gm30, 3 cylinder big sister to your engine) and the exhaust mixing elbow was almost completely choked with carbon.

Might be worth checking.
Good luck,
Andy

Current owner was under the boat this weekend (with scuba gear) to switch props. Think he would have seen a crab pot if one was present. In this area (Chesapeake Bay) you have to be real careful, or you will end up with one.

Will check the mixing elbow and water trap. Will also (carefully) check the air inlet. Wasps have been known to take up residence there.

One other bit of information, the engine has about 100 run hours since rebuild, most running well, so it is unlikely to be a bad assembly issue.

air filter OK ?

Did the problem start before the prop change or after. Could be different pitch prop.

I would agree with this. We used to service Onan generators and this was a fairly common problem. The exhaust is raw water cooled at the exhaust elbow and they corrode after 4-5 years of service. If you dropped 3" of pitch and the rpm's still didn't pick up your engine is not making power. I wasted about 40 hours rebuilding an onan, chasing the fuel system, timing etc to learn this one.

When I had the boat, I had a 14x14 folding prop on it, once the new owner started to have problems, he went to a 13x9, which didn't improve the problem, and now he is going to go to an even smaller prop (don't know why).

Like I said, when I had it with the 14x14, and the engine was running properly, it would push the boat along at 3000 rpm at hull speed (let's just call that about 6 kts). Going down to a 13x9, the engine should have been able to spin up to redline, and have the boat poking along at 3.5 kts. It can't. It starts to bog then die at 2100 rpm.

This is not an uncommon problem with marine diesels. There are several steps to proper diagnosis:

i. Did it ever run properly AFTER the work on the injector pump & replacement injector? If not, it is possible that the injector timing is wrong. e.g. timing off by 1 gear tooth or similar.

ii. The black smoke suggests the engine is overloaded and/or getting too much fuel. What happens to the engine temperature at the point when it bogs down & dies? Could it be overheating due to overload? Is the correct nozzle installed on the injector?

iii. Bogging down & dying under load can also be caused by fuel starvation, although this is not normally accompanied by black smoke. Try running the engine on fuel from a jerry jug, thereby eliminating fuel line blockages. Also check the lift pump for proper operation.

If you still have no luck, I'd recommend getting a certified Yanmar dealer to work with it. Plenty of them in the Annapolis area, otherwise you could come over to see my colleagues at Oxford Boatyard. We're a Yanmar Gold Dealer. (410) 226 5101 M-F 8:00-4:30

My gut feeling is this is a drive line problem. Split the coupling from the gearbox and check the alignment. Could be that the motor mounts have settled or gone out of adjustment since the rebuild. Yanmars had very soft mounts. Next step would be to draw the shaft and check for wear areas and inspect the strut bearing.
Several years ago I did some long distance (So Cal to Greece, via fax and snail mail through the Bulgarian embassy!) trouble shooting of a similar problem for one of my former marine diesel students. It was the same symptoms as described above, going so far as to stall the engine. Bottom line was gross misalignment that wore the cutlass bearing right through to the aluminum stern tube. She sent me the remains tied up in a red ribbon to show my students.

With the engine shut off, you should certainly be able to rotate the coupling and shaft freely by hand. If not, there is something bad afoot. e.g. fishing line or small dia. cordage ingested into the cutlass bearing.

Check if any rubber hoses are in Air intake...
Might be getting warm and collapsing.
Black smoke means too much fuel (could be sudden overload tho) not enough air.

Also check fuel return lines for collapsing/sharp bends...

I know the engine and shaft are aligned, I repacked the stuffing box for him, and made sure that the flanges were parrallel and centered.

Shaft turns easily by hand when in neutral.

No rubber hoses or ducts on the air inlet, just a sheet metal filter housing bolted directly to the intake.


Problem started before the fuel injector pump rebuild and new injector. I took the boat about 2 miles to a marina, he brought it back, with no problems. Two weeks later the problem was there. Not a slow progression, but overnight.

I still need to get on the boat and check the air inlet, and exhaust for obstructions, and check the fuel line as well.

Thanks for the suggestions.

I'm not familiar with the Yanmar engine that you have trouble with but we have had some problems with Volvo Pentas failing to achieve any decent sort of output caused by the fuel pump drive slipped enough to run OK at low power but not at high. Symptom was black smoke etc. Also is the fuel pump correctly calibrated and is the injector, though new, set correctly for your engine?

Good luck

I had an experience with a Universal Atomic four engine years ago where the exhaust water jacket had frozen and partially collapsed the exhaust pipe. Engine would idle but die under load. The drain for the water jacket had been hidden behind some teak trim.