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Old 07-06-2020, 12:04   #1
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Clutch too far gone? Mercury 3.5hp Outboard

Hoping for some of that Cruisers Forum education here. Our dinghy engine (Mercury 3.5hp 4-stroke outboard) stopped engaging in forward. We opened it up and found all the shifting to still be intact (looked good even), the only piece I've found that is less than good looking is the small metal clutch in the propeller drive shaft (part #7 in the diagram). I attached two photos of that clutch too. It has a small amount of wear on opposite corners. It make sense to me that the wear could make it slip while in gear. Could it make it completely miss gear altogether? When the engine is engaged in forward it operates the same as in neutral now (despite the shifting absolutely engaging in the propeller drive). No extra noises, no engagement of the prop. What do you all think? I'm going to order the replacement clutch, but I'm worried there's something else upstream that I'm missing. No idea how the clutch got worn down in the first place. That would've required our flimsy plastic prop to stop moving while the flywheel was still turning right? Could old oil in the lower unit have been the issue? It was pretty black...Any insight the forum can offer I'll take. Learning along the way here but I know anything I've run into has probably been dealt with multiple times by individuals here. Thanks for being such a resource!
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Old 07-06-2020, 12:08   #2
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Re: Clutch too far gone? Mercury 3.5hp Outboard

By any chance is there a rubber "clutch" in the propellor hub that is slipping?

Many of these engines have such a hub to protect the drive train if the propellor hits a rock, or is otherwise overloaded.
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Old 07-06-2020, 12:57   #3
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Re: Clutch too far gone? Mercury 3.5hp Outboard

Sparx thanks for checking. I did open up the prop driveshaft assembly and found a sheer pin (part #15 from the diagram), which was in fact sheered. Could that sheering have led to further clutch damage? Or you think these are two unrelated issues? No engagement of prop because of sheer pin makes perfect sense, but before that happened our outboard was stalling out after ~30 seconds of running when put into gear. Could the clutch slipping cause the engine to stall out then? Oh the mysteries of outboards!
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Old 09-06-2020, 06:28   #4
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Re: Clutch too far gone? Mercury 3.5hp Outboard

Clutch dog looks like it'll be fine.

Does the engine still stall after changing the shear pin? If not, I assume the sheared pin was allowing the prop to move forward and rub against the gear housing, causing enough friction to stall the engine.

Shear pins don't usually, unless one hits something. Did you? That the 'damage' on the dog is only on the ends would seem to indicate it happened in normal use, but anything is possible, I suppose.

Don't shift the engine until it is at idle.
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Old 10-06-2020, 07:04   #5
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Re: Clutch too far gone? Mercury 3.5hp Outboard

jimbunyard, that would make sense, I ordered the replacement pieces so we'll see.

I don't think I hit anything with it, certainly didn't notice. I'd be surprised if the little plastic prop on the engine would survive a strike without any sign of damage too, and it looks fine.

The wear on the clutch coming from high rev shifting makes sense too. I've had difficulty getting this engine to idle most of the time we've had it (despite adjusting idle & mixture screws), so it was often shifted into gear at higher than idle.

The only problem I have with the shear pin being the start of the issue (chicken and the egg here) is that if it broke shouldn't the propeller have stopped engaging then and not much later? I'm going to drop the lower unit while I'm waiting for these parts and see if I can find some other source of friction/resistance that might have caused the stalling.
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Old 17-06-2020, 08:32   #6
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Re: Clutch too far gone? Mercury 3.5hp Outboard

Just to wrap up this post, this turned out to be a series of issues. Our engine stopped spinning the prop because the shear pin broke. Easy fix, no big deal, thanks to Sparx for pointing me in that direction. Our stalling out in gear was either a function of a dirty spark plug (which I swear didn't look so bad before, but maybe all these short runs helped it get dirty?) and a leak at the carburetor gaskets. Whatever the case is, after greasing the gaskets and replacing the spark plug the dinghy is back in action. Really appreciate this forum as a spot to get some experienced advice!
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