Wanna help brainstorm an O/B problem?

[hart]

My outboard is acting up and I'm not quite ready to throw in the towel - but I'm close. The mechanic can't get to me until next week at the earliest so I'm trying to troubleshoot it myself.

Wanna help me brainstorm?

Here goes: It's a Mercury ELPTO 50 (the classic 50, not the bigfoot). At idle it sounds fine, in the backyard (no load on the prop) it sounds fine but on the water in forward and reverse it makes a very regular "knocking" sound and tops out almost 10 mph less than it did last season with the same prop. Okay so at first I'm thinking something in the powerhead. I checked all three cylinders tonight and they all have good compression, right in range and within 2% of each other. Nothing obvious seems wrong that I can see up there either.

After playing with it some more tonight I think maybe the "knocking" is actually in the lower unit but it's hard to tell in the back yard since it doesn't do it without a load on.

More info? Hmm, I hit a submerged 4x4 post and tore up a prop at the end of last season. I'm not positive but after thinking about it some more, the "knocking" probably started not too long after that.

The prop seems to seat properly on the shaft, the nut and tab washer hold it just like they always have.

When I changed out the impeller everything seemed normal in there, I think, but I wasn't looking that hard since I assumed it was the powerhead at that point.

There were a few metal flakes in the lower unit oil but not bad - like tiny glitter. I couldn't feel them in between my fingers, and honestly didn't even see them until I noticed my shorts "sparkling" in the sun after I had wiped my hands off on them.

Anybody got any good ideas of something to check before I call my mechanic back? I've been trying google but "lower unit knocking" brings up more crap that you would believe and I have no idea what other terms to search for.

[Norca]

Any vibration when running?
I would open the lower unit and have a look.
If it is running fine on idle, and start knocking as soon as You put it in forward or reverse, there is
a problem in the drive shaft or lower unit, if it was only forward or reverse, it could be the gears in
the gear case.

[hart]

Vibration, no not really. You can feel it in your feet at WOT but it's always been like that.

Oh forgot one other thing. It should be hitting 5500 RPMs at WOT (and did with this prop last season) but now my tachometer is showing over 6k at WOT. Now that may be related or it may be a separate issue. I haven't investigated it yet.

[yukonbob]

Everything sounds like lower leg. The high rpm would also explain if not fully engaging or slipping.

[restless]

does your prop drive through a shear pin or shear-bush. I've heard of the odd bit of randomness after a strike. If the drive cushion/bush bit has been wrecked the prop is not properly attached any more. Stick it in gear and see if you can turn the engine over, or maybe even lock it on the starter ring and check that you can put a load of body weight on the prop without slip.
Good luck!

[hart]

Ok. Cool, a couple of things to try. Hopefully I'll get off work a little early today and can go mess with it some more. I'll let you guys know what I find, if anything. Thanks for the advice all, that's a big help.

[Catigale]

Hart

You can also engage the gearbox at idle and see if you can I isolate the noise to powerhead or gearbox. Just keep the engine at idle at watch out for the prop!!!

[Don T]

Hello,
Sounds like the hub might slipping. You won't be able to slip it by hand until it gets worse, think about how much torque it takes to make it slip under power now immersed in all that water lubricant.
Another possibility, one of the prop ears is bent causing cavitation. You might not notice a slight deformation. If you have a spare prop of known parameters give it a test or cut out a contour guide and compare the ears. If the spare works well at least you know it's not the power head.

[Hamin' X]

My vote is for a spun prop. When you hit the 4X4, the prop stopped and the hub did not.

~Rich

[DaveB]

Props. Hubs are made so if one hits something the hub takes the pressure and not your gears. You may of damage the hub but not bad enough to limit you to around 6 mph, enough to get you home. This may be the reason your getting 500 more rpms.
I would replace the hub and if that doesn't work I would ck the control gear cable adjustment, it could be both Cable adjustment and a damage hub but I would bet it's the hub.
Also use a magnify glass for any tiny cracks on your prop.
Also ck your lower end gear lube for water, oil will look a little like milky.
Dave

[hart]

Talked to a motor guy today at the store and he gave me a few things to try in order to isolate the noise. I was hoping to play with it tonight but I got off work too late. So tomorrow is another day.

Ok, oil is not milky so that's cool.

The hub. Hmm. I tore up my "spare" prop on the 4x4 so I put my regular one back on there after that. But seeing as how I need another spare anyway I think I'll pick up a new hub and prop at the store tomorrow and try that. You guys may be on to something with the hub.

Catigale, that's a good idea and one the motor guy suggested too. That's at the top of my list.

Thanks all, I'll report back.

[hart]

Took apart the lower unit tonight and all looks well. The prop is fine too I believe.

So I put the outboard in gear while running and without the prop attached and poked around and listened. It sounds to me like the vertical drive shaft (no idea what it's called) is clanking around loose just a couple inches under the power head. So I'm guessing that means there's a coupling or something in there that's stripped out. I have no idea where to attack that from so I think it's time to call in a pro. I've done about all the damage I can do I think.

[Catigale]

Ugh. Sorry to hear tHat hart. Have you priced a lower unit replacement ?

[hart]

No, and I'm scared too. I'll have to start scrounging around. Gonna be time to sell some stuff I guess. Hmm, I have two 40 gallon aluminum below decks fuel tanks. Anybody wanna trade, .

[Dido]

Reading the Seloc outboard manual the prop as a rubber hub shock absorber that is set to slip above any normal propulsion force ie when it hits something, the drive shaft should continue to turn with the hub slipping while the prop is stuck. If it becomes worn it can slip under normal operation and you loose performance as it slips rather than driving, this also allows the drive to spin at greater rpm due to the prop giving less resistence. If your drive is damaged it might also be worth getting the prop shock hub replaced. Good luck