Droopy yanmar

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cameocrew

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Mate of mine got problem with his Yanmar 3HM. Engine stalls when put into drive.
Engine and gearbox (Hirth) has had shed loads of money spent on it-ie injectors/fuel.pp/compression test/mountings/control cables/new drive plates in box/etc- shafting /flex coupling and stern gland o/hauled. Engine runs as sweet
as nut at idle (850rpm) and in drive ,but still stalls going into ahead or astern.
Idle rpm has been raised to 950-still stalls. Engine disconnected from shaft-still stalls. I have had a look at governer system (drawings) and figure that anyone
who can design a system like that must be mad / american (sorry- to many xcelent USA engineers)/or a woodbutcher. Cannot figure it-advice required.
 
You tried turning the shaft and prop by hand and say all goes round OK, but is that with the gearbox in neutral ? What if the box is the problem, and is locked up (despite, the new bits) ?
 
If the engine stalls when the gearbox is put into gear although the prop-shaft turns "v freely" when the gearbox is in neutral then it would appear to me (initially) that the gearbox is at fault.

If it was my unit I would disconnect the gearbox from the engine and try again or if possible try turning the gearbox input shaft with the box in neutral then again in gear (whilst disconnected from the engine).

In any event if the engine runs without the gearbox and revs when the throttle is opened I would conclude the gearbox was at fault UNLESS the slight drag of the box is enough to cause the engine to stall, in which case you could have a governor problem but I would favour the gearbox to be at fault.

I must admit I am a little puzzled though as you said the propshaft turns v freely when the box is in neutral so one could conclude the gearbox output shaft is O.K. (turns free). As the input shaft is driven by the crank shaft one could conclude the input shaft is O.K. (turns free, engine ticks over in neutral) /forums/images/graemlins/confused.gif

The only reason I can think off for the box to then lock up in gear and stall the engine would be if both forward and reverse clutches engaged at the same time. ....... /forums/images/graemlins/crazy.gif /forums/images/graemlins/confused.gif

Back to the engine then. The only extra load on the engine should be the drag of the box and the prop/prop shaft which according to you is mimimal but, maybe it is enough to drop the revs enough to stall the engine before the governor kicks in to allow more fuel through.

Have you tried increasing the tick-over slightly?
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Check there is no blockage in the exhaust or air inlet (would need to be almost fully blocked). Just a thought...

Andy
 
Cliff's analysis appears to be pretty sound. Maybe you can get some further input by removing the injectors (assume you have no decompression lever) and rotate engine by hand, then put it in gear to see if this brakes the engine. Or you might be able to do something similar by rotating it on starter motor with the fuel turned off, but watch out for flooding the engine with cooling water.

It does sound very much like a fuel supply problem to me. I remember an example when we could rev the engine as much as we wanted without load but it absolutely refused to pull even at low revs when in gear.
 
Nice one! I didn't think of decompressing the engine (or removing the injectors) and trying to turn the engine in gear to check the gearbox /forums/images/graemlins/confused.gif
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hammer.thumb.gif
"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>
 
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