Yanmar Overload ?????

[TeamSpirit]

Anything up to 2500 revs and my engine is behaving perfectly. Throttling up then creates volumes of thick black smoke and is leaving a black deposit on the surface of the water. The engine revs remain relatively static if throttle is pressed any further than 2500. No other symptoms and when engine in neutral no symptoms experienced.
Not that I fancy going over the side but I suspect that I have aquired something on my prop which although is not seriously snagging is causing the engine to be overloaded at speed ?
What do my learned forumite fellows think ??

 

[Salty John]

Could be. Check your air filter isn't clogged, and that your exhaust system isn't partially blocked before taking the chilly dive.

 

[ninnes]

I suspect your hull is seriously fouled or your prop is encrusted thereby preventing revs rising when in gear. This causes incompletely burnt fuel being pushed into the exhaust on onto the water. This explains why things are fine in neutral.
Looks like a trip over the side is needed!
Nick

 

[RupertW]

I had exactly that last year and assumed it was a prop fouling problem but it turned out be a problem with the injectors which must have got clogged by something which had managed to get past the filters. The amount of black smoke was horrible and the injectors needed to be sent off and either replaced or fixed - can't quite remember.

 

[savageseadog]

This can happen if you overload the engine perhaps with too large a prop or you might be fouled.
Basic principle is that the more fuel you inject the faster the engine goes. If the engine can't go faster the fuel won't be burnt due to lack of air, hence the black smoke. Other suggestion of checking the air filter is sensible, try taking it off and running without for a short period. Can also be poor injection.

 

[wiggy]

Had exactly the same 2 weeks ago and it was a large lump of weed around the prop, go for a dip and take a look.

 

[andy_wilson]

incompletely burnt fuel ?????

Incompletely burnt fuel will be blue grey and stink to high heaven. This is due to a poor injector pattern usually.

Black smoke and a sooty transom is carbon produced by fuel completely burned, but with insufficient oxygen. The black bits are carbon that are left over when ther is not enough O2 to make it into carbon dioxide.

Pedantic I know, but it is important to be able to differentiate before the boatyard starts charging money for tackling the wrong problem.

I agree with your diagnosis however, though I would check the air filter before taking a dip, and also that the engine compartment isn't sealed up too well. Plenty of saily boats can develop black smoke after a serious round of sound insulation!

 

[savageseadog]

Re: incompletely burnt fuel ?????

[ QUOTE ]
The black bits are carbon that are left over when ther is not enough O2 to make it into carbon dioxide.


[/ QUOTE ]
Being even more pedantic the black soot is incompletely burnt fuel, what else can it be?

 

[srp]

Re: incompletely burnt fuel ?????

Exactly the same happened to me, and it was a polythene bag. Amazing how tightly wound on it was, but it only took a few minutes to sort out.

 

[TeamSpirit]

Re: Taking a dip

I have already checked my air filter and although I had improved my sound insulation during winter lay up I was careful to ensure adequate ventilation and left my air filter off and engine cover up to enable me to isolate the problem. No difference.
I am obliged to you all for your helpful advice.

Looks like i'll be digging my wet suit out and taking a dip in the marina. Yippee !

Many thank guys. I'll let you know the outcome (once i've got out of quarrantine !*?! .

Cheers

 

[andy_wilson]

Re: incompletely burnt fuel ?????

No no no.

The burning of a hydrocarbon in an engine is a chain reaction catalysed by heat, first separating the components and then reacting with oxygen liberating more heat to expand and drive the piston.

If the fuel is incompletely burned, there would be whole, unreacted fuel ejected from the exhaust. This part of the fuel has taken energy from the engine and not given anything back. This would be vapourised, smelly, blue / grey in colour and a symptom of a particular type of fault.

If the fuel burns with a sooty exhaust, the fuel IS being completely burned (broken down into contituent atoms and molecules, and re-combining with oxygen to release energy), but with insufficient oxygen (O2) to convert all the HydroCarbon fuel into CO2 and H2O. As the hydrogen component is most highly reactive, this would normally manage to attract the lions share of O2, leaving some of the carbon fully reacted to CO2, and the rest liberated as pure carbon particles. In this case it is not the fuel that remains unburned, but the carbon.

All highly simplistic as there are plenty of other reactions taking place, and plenty of other waste products in the exhaust.

However, sufficient to say that in the latter case, the fuel does get completely burned (or reacted would be a better term), provides drive to the piston, but the chemical formula is different due to the lack of O2 on the left hand side of the equation.

Furthermore, in the latter case, this is a symptom of a very different fault.

 

[jfkal]

Re: Taking a dip

Now now. It is summer so take the dip rather sooner than later. Change the zinc anode whilst down there. /forums/images/graemlins/grin.gif

 

[MASH]

Did this problem develop or has it always been there? I have similar with my 3GM despite new injectors, no insulation and a clean prop. I think my trouble is being over-propped, and tho I still get almost 6 kts (nigh on hull speed)at 2100rpm I cannot get more than 2500rpm. Black smoke over 2400. I will look up prop spec and report back.

 

[Marsupial]

Mash

My last yacht had a 3gm, you should be looking for 3600rpm flat out, at hull speed and no black smoke. Sounds like a lot of over prop, not a good idea for a long engine life. That said the 3GM smokes like a trooper when you increase revs rapidly, clouds of thick black stuff comes out and it leaves a trail of soot in the wake, I have observed lots of them doing this but the black smoke should go as soon as the engine revs stabilise.

Seriously a few bob spent the right prop for the engine, gearbox, hull profile will pay dividends.

Cheers


David

 

[davidbains]

I'm sure a lot of Yanmars are overpropped.
Not least because a 2GM20 does not produce 20hp and a 3GM30 does not produce 30hp,
even at high revs. At lower revs the power output is well down.
I had to fit a very fine prop to get my 2GM20 to rev up.
The new YM's are slightly more powerful.
Just lashed out on a Kiwi prop for adjustable pitch, and feathering.
Will be able to report results in two weeks.