Black Smoke- explanation sought

[nick8489]

First basic facts:
-Yanmar 3YM30
-10 years, 1300 hrs
- Hunter 33 2004
- Had been running perfectly and could get full revs.

Went out Monday to set up new auto pilot. Cruised in three circles at 2 knots engine around 750 revs and took about 15 minutes.
Headed for home and could only get 2000 revs. looked over shoulder and masses of dense black smoke. Took it out of gear and revved up in neutral; still masses black smoke so nothing round the prop.
Cruised back to berth.
Convinced myself it was exhaust elbow.
Took it off and clean as a whistle ( it's a cast aluminium one, brilliant compared to Yanmars cast iron disaster!)
Put everything back together again and by now it was dark, but started motor up again and although it was very dark a flashlight on the water indicted a stream of black substance coming from the exhaust.
Went home to contemplate, had a couple of beers and convinced myself that the engine was buggered.
Returned the next day fired her up and no apparent smoke. Took her out for a blast around the bay and everything sweet as pie, no smoke, full revs!
Has anyone out there got a logical explanation for this?
Could crap accumulate in the exhaust system after 10 years and then pop out like a cork!???

 

[KD3PC]

air cleaner or restricted air flow is the usual reason for black smoke. INjectors or injector pump are the next possibility.

start simple with a good visual inspection to make sure the air path is correct.

 

[jibes138]

Two thoughts, too much fuel from the injectors, or oil leaking past valve guides or piston rings. Could the injectors have been stuck a bit causing an overly rich fuel mixture? Think of a big diesel truck blasting out black smoke on the highway especially when accelerating. Maybe after sitting for a while with fresh fuel it loosened up what was sticking and started running clean again? I'm far from a diesel mechanic so let's hear from those that are.

 

[Richard Bryer]

Check your air cleaner- if you have one of those foam cones, it may have disintegrated and parts of it clogged the intake.

 

[Scott T-Bird]

Hey Nick,

It sounds like a temporary blockage of air that cleared out. Check filters ... air and fuel.

BTW, my middle son is on a trip to Brisbane as I type! He left SF last night and may be in Fiji about now, I suppose. He's going to see his girlfriend. She had to leave the States when her visa was up, and he wasn't able to get a work visa in Oz so he is staying for just a month. They'll have to figure something out ... she is a sweetheart for sure! She landed a great job in Brisbane as soon as she got back last October. She is from Cairns. We were talking about sailing in Brisbane just last night.

 

[Benny17441]

That frantic mouse in the intake tube was finally able to escape under the darkness of night. Don't worry about it until it happens again.

 

[woodster]

That frantic mouse in the intake tube was finally able to escape under the darkness of night. Don't worry about it until it happens again.

benny i like that one ...i can see speedy gonzales hanging on for dear life

 

[Bill Roosa]

Ok so black smoke means to much fuel or too little air. The good news is compression is ok because it is in fact burning, just not the way you want. So rings and valve seals are probably OK.
elbow is OK so you "should" be getting the exhaust out to make room for the intake air. Still want to check the ENTIRE exhaust system though if you don't find an intake problem.
The other thing is how long have you been in the water? lots of hull drag can result in a under reving and over fueled (for the rpm) engine. Try the rpm test in neutral.

 

[nick8489]

Check your air cleaner- if you have one of those foam cones, it may have disintegrated and parts of it clogged the intake.

Thanks Richard and everyone who mentioned the air intake.
In fact around year 3 I inspected the cone type air filter and it had already started to disintegrate. In my ignorance I went to the local Yanmar agent for a new one and he didn't stock them as he said nobody uses them and the air in the engine compartment is clean enough. So it has been running for 10 years without one!

 

[nick8489]

It sounds like a temporary blockage of air that cleared out. Check filters ... air and fuel. BTW, my middle son is on a trip to Brisbane as I type! He left SF last night and may be in Fiji about now, I suppose. He's going to see his girlfriend. She had to leave the States when her visa was up, and he wasn't able to get a work visa in Oz so he is staying for just a month. They'll have to figure something out ... she is a sweetheart for sure! She landed a great job in Brisbane as soon as she got back last October. She is from Cairns. We were talking about sailing in Brisbane just last night.

Hi Scott,
You note deserves a reply!
Firstly I agree it must have been a temporary blockage and I am investigating, but more importantly:
If your son and girlfriend want a sail in Brisbane come down to the Royal Queensland Yacht Club any Wednesday afternoon (12.00 noon) when we have WAGS (Wednesday Afternoon Go Sailing) and all visitors are allocated a crewing position on one of the fifty or so yachts who take part in a handicap race. I am there most Wednesday's so they just need to ask for Nick or "Sweet Mango".
PS All the girls from Cairns are Sweeties!

 

[nick8489]

That frantic mouse in the intake tube was finally able to escape under the darkness of night. Don't worry about it until it happens again.

Love it Benny!
Will go to the hardware store to get a mouse trap this very instant.

 

[nick8489]

Ok so black smoke means to much fuel or too little air. The good news is compression is ok because it is in fact burning, just not the way you want. So rings and valve seals are probably OK. elbow is OK so you "should" be getting the exhaust out to make room for the intake air. Still want to check the ENTIRE exhaust system though if you don't find an intake problem. The other thing is how long have you been in the water? lots of hull drag can result in a under reving and over fueled (for the rpm) engine. Try the rpm test in neutral.

Thanks Bill for you considered reply. Re hull drag ; as we sail 12 months of the year in Brisbane the boat only comes out of the water for a couple of days every second year for anti fouling. In between times there is diver service for $70 who does a bottom scrub every three months. Mine was last done only three weeks ago. Next time down to the boat this weekend I will be looking for any evidence of a temporary intake blockage (or benny's mouse !) and after blocking up the exhaust outlet from the hull checking through the exhaust system.