Yanmar 2GM20F blew black smoke

[stuendan]

Hi all - The Yanmar 2GM20F in my 2003 Hunter 326 may be unwell.

In an attempt to sail 5 nm from my marina to another, for a haul out appointment, I was punching into 25kts and 2kt tide, and 1m waves (3'). Thumping along I couldn't get more than 2300 rpm and 2.5 - 3.5kts OTG. Looked around and was alarmed to see black smoke and oily residue coming out of the exhaust! Temp and oil pressure both ok!
I pulled the plug and returned home (at low revs with tail wind). All seemed normal on way back (for my boat)
Today, in the marina, I got 4000rpm in neutral flat out. From manual, the motor develops max power at 3600 rpm
Tied up, I could only get 1800rpm in forward at full throttle but no smoke noticed in the very short time I did this test.
I have never got much over 5kts under power in flat water since I've had the boat

I am aware that the smoking under load is caused by poor combustion and with this motor, usually means a partially blocked mixing elbow in the exhaust (I took off air cleaner to eliminate intake restrictions and no difference noted).
Before I bite the bullet and order a new mixer/riser, just wondering what sort of revs and performance under power I should expect from this boat.

Thanks all

 

[kappykaplan]

Although I don't have a Yanmar in my boat, the symptoms you describe are eerily similar to the symptoms I had when my mixing elbow got partially blocked.

 

[kloudie1]

A fouled propeller will also do that. Check that first then check the elbow before ordering one.. Good luck.

 

[Ralph Johnstone]

With the right prop, cruise at 5 kts at 2800 RPM. Don't buy a new mixing elbow until you inspect the old one.

This was a mess but hadn't caused any noticeable problems YET.

What was your performance before the rough ride ?

 

[Benny17441]

Overfueling. The governor is unable to handle certain load conditions. Fouled bottom or prop or restricted exhaust or air intake. Sometimes bad fuel can be the culprit. Having the engine freely rev in neutral does not point to any particular cause. Check everything leaving the mixing elbow for last.

 

[stuendan]

Thanks for the input guys,

Boat comes out for antifoul and Prop Speed next Monday so I will see how she goes then. As usual in life, it's probably a combination of things, including dirty bottom/prop and partially blocked elbow.

 

[Benny17441]

Sometimes RPM irregularities are a result of a faulty Tachometer. The fact that you could propel the boat at 3.5knots against a current of 2 knots denotes a speed over the water of 5.5 knots which is respectable for the underpowered boat. (we had an h320 with the 2GM20F and it was a dog under power). You seem to indicate the engine has ran fine after that, with no smoke, except erratic RPM readings. Check and calibrate the Tach. We could hardly get our h320 to exceed 5 knots under power. Go ahead and clean the bottom and clean and balance the prop if needed before replacing the mixing elbow.

 

[Ralph Johnstone]

The fact that you could propel the boat at 3.5knots against a current of 2 knots denotes a speed over the water of 5.5 knots

Bloody spell check of yours changed "speed over land" to "speed over water".
They always do that.

 

[Dalliance]

Black smoke and oily residue on the water could just indicate unturned fuel because you had the throttle wide open.

 

[BigEasy]

Had a 2GM20F that suddenly produced heavy black smoke on startup. Ran fine with no smoke after it warmed up. Dock mate suggested that I use an injector cleaner fluid and I did. After a month or two the problem stopped completely. Never really determined the cause and if the injector cleaner was the solution or just coincidental.

 

[stuendan]

My Hunter 326 is now out of the water and everything is pretty clean with only a little bit of crud on the prop. On way up to the marina, in flat water, I managed 2800 rpm at WOT and 5.5 kts GPS (short time and no smoke).
Now the manual states that the Yanmar 2GM20F is rated for a continuous 16 HP @ 3400, and a 1 hr rating of 18.2 HP @ 3600.. Everything I've read from boat tests and feedback indicate that you should expect 6 kts at 3000 rpm in this boat and motor with standard prop. With the specified motor performance as above, shouldn't we be looking to get more revs at WOT?
I have an after market feathering prop (Seahawk Autostream S1 from Australia) which I have discovered allows for a relatively simple pitch adjustment. The prop specs says pitch adjustment gives 150rpm per half turn on adjusting bolt, therefore I look like needing 4 full turns. I shall get on to the manufacture to see if it would have been set up at the factory to suit this boat/motor. I will also attempt to pull the exhaust hose off the elbow and try and see how coked up it looks up there!

 

[RoyS]

I have a different reversing, feathering prop. When my pitch was too steep I had the same symptoms. It took me several trial and error attempts to get the correct pitch. It certainly sounds like the pitch setting is your only problem. You are almost there.

 

[Ralph Johnstone]

I will also attempt to pull the exhaust hose off the elbow and try and see how coked up it looks up there!

Whole elbow needs to come off and sit on the work bench for an inspection. Don't waste your time.

It certainly sounds like the pitch setting is your only problem.

@RoyS wins the kewpie doll.

 

[AndyBC]

The 2GM20F on my Hunter 28 (standard fixed prop) pushes the boat along at 6kts at around 2800 rpm. WOT gets me to 3600 rpm (though the boat doesn't go much faster).

My Hunter 326 is now out of the water and everything is pretty clean with only a little bit of crud on the prop. On way up to the marina, in flat water, I managed 2800 rpm at WOT and 5.5 kts GPS (short time and no smoke).
Now the manual states that the Yanmar 2GM20F is rated for a continuous 16 HP @ 3400, and a 1 hr rating of 18.2 HP @ 3600.. Everything I've read from boat tests and feedback indicate that you should expect 6 kts at 3000 rpm in this boat and motor with standard prop. With the specified motor performance as above, shouldn't we be looking to get more revs at WOT?
I have an after market feathering prop (Seahawk Autostream S1 from Australia) which I have discovered allows for a relatively simple pitch adjustment. The prop specs says pitch adjustment gives 150rpm per half turn on adjusting bolt, therefore I look like needing 4 full turns. I shall get on to the manufacture to see if it would have been set up at the factory to suit this boat/motor. I will also attempt to pull the exhaust hose off the elbow and try and see how coked up it looks up there!

 

[Dean Strong]

Almost certainly is mixing elbow. I'm on my third (Yanmar 2GM20F, 1990 H30). Manual says to inspect every 300 hours. I always have, and they have always been coked up, at least partially. Sometimes you can salvage, sometimes not. Check out stainless elbow from outfit in Vancouver, WA. Doesn't corrode, which contributes to the problem. Get a hose pick to slowly pry away exhaust hose, be patient, use WD40 to spray and let it soak in. That's the hardest part of the job. Once the hose is free, the elbow & flange comes off easily together. The elbow is connected to the flange by a coupling, top half on the elbow is reverse thread. I usually have to but the flange in a large vice and hit the elbow with at least a 5lb hammer to get them apart (always use anti-seize compound) Be sure and re torque to spec on reinstallation, and don't forget a new crush washer. I always carry a spare.

 

[Dean Strong]

Also...wide open throttle should get you at least 3600 rpm. I cruise my H30, same engine, at 2900 RPM; 5.5 knots with clean bottom. Wide open throttle should produce some black smoke, back off 100 rpm at a time until there is no smoke, and you can stay there as long as you want. Every shut down, after 5 minute very low rpm cool down, slowly increase RPM in neutral to WOT, then moderately back down to idle and immediately shut off. Will keep elbow cleaner longer.

 

[Ralph Johnstone]

Get a hose pick to slowly pry away exhaust hose, be patient, use WD40 to spray and let it soak in. That's the hardest part of the job.

Liberally coat the ID of the hose with parting compound and it comes apart, at least, easier than without.