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Old 30-04-2010, 06:45   #1
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Loss of Power

My Yanmar 20GM stalls when put in gear. Will not even try to rev up. It worked fine last year. All we did this spring was to put as new cutlass bearing in. Could the martec prop being assembled wrong and cause a 12 HP diesel to stall out?

The engine revs up fine in neutral. Shaft spins freely, in neutral. Tranny goes into gear easily. When shifting into gear, the engine loads up. When giving it more fuel, it will not rev up, just blows black smoke and stalls out. Upon starting again, it belches out raw fuel.

I do not have a neutral safety switch.
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Old 30-04-2010, 07:38   #2
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Process of Elimination-black smoke

Put it in neutral and try turning the prop by hand, if free, that'll eliminate the cutless bearing but don't rule out the trany.

Put it in forward gear while running then shut down while in gear, then check the prop position.

When were the injectors inspected last?

What's the results?
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Old 30-04-2010, 07:46   #3
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Shaft turns free when in nuetral.
Injectors cleaned about 5 years ago. (Engine ran fine last year.)
Not sure what to look for by stoping the engine in gear. Boat is in water, and the water is real cold still here in michigan. Is it possible that the prop could have a pitch soo large to stall the engine out at low RPM?
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Old 30-04-2010, 10:19   #4
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Could there be some flotsam, a line and bouy maybe, wrapped around the shaft out of sight? So when it spins up fast, it snags and stalls out the engine?

Bearing in mind cold water and lift prices...I'd still think it might be most effective to dive or haul, and if there's nothing wrapped on the shaft, remove the prop to see if the engine and tranny alone have a problem (i.e. a tranny problem). Kind of a blunt brute force approach but it will eliminate any prop questions fast.
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Old 30-04-2010, 10:29   #5
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Is it safe to assume that this problem occurs in both forwards and reverse? Will it stall at lower throttle settings or only higher throttle settings?

The way that you describe it, it sounds like you are putting a large load on the engine. The suspicious part to me is that the engine stalls when you try to rev it up (I am assuming that it doesn't stall at lower throttle). It is possible to overfuel so heavily as to cause the engine to shut down but that is pretty extreme and would cause a very large load.

Possible loads could be from the prop being severely overpitched, running in a slip with a large current from astern, a fouled prop/shaft (line or fishing line or something else), a binding shaft (does it wobble or vibrate in gear?), or an internal transmission problem.

Before hiring a diver or hauling, you could detach the shaft from the transmission and try revving up in gear that way. If it loads down, you need to suspect a tranny problem.

Good luck.
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Old 30-04-2010, 10:50   #6
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THanks all,
I thought about a fowled prop, but it turns freely by hand and when the engine out in gear it does spin and makae no thumping souond that would indicate a foul.

I will try the disconnect the prop shaft from the tranny and see what happens when put in gear while running.
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