Mercury Classic Fifty Cavitation??

[tsp1158]

Hello! I have a bit of a unique problem & question:
I have a 1986 Mercury 45 horse outboard, and we just dewinterized it after 2 years sitting in the yard.
I always do a good job fogging, changing the lower end lube, etc. and the motor started right up.
I ran it for about 30 minutes with the muffs & garden hose, and have no problems.
When we put it in the water all went well for the first 10 minutes, until I brought the motor up to about 2500 RPM's. At that point, the motor seems to "slip" and rev to 3000 + RPM's.
All seems fine if I stay at 1500 - 2000. It does the same whether in Reverse or forward.
I pulled it out of the water yesterday, and there is no damage, I haven't hit anything etc. When off, if I put the motor in gear and turn the prop I see the fly wheel turn, so there is no lower end slippage, etc.

My question is, has anyone seen anything like this?
I'm guessing maybe a wasp / bee colony may have built / clogged something diverting exhaust causing cavitation.
Does this sound reasonable? is there any way to back flush or prove this theory?
Any other suggestions ?

Thanks! - Tom

 

[Texasmark]

Re: Mercury Classic Fifty Cavitation??

Slip and rev is prop slippage if you are running this on the same boat with the same rigging as you have done in the past.

Only thing I can think of is the rubber in the prop (slip hub) dried out and shrank during storage.

Pull the lock key and nut off the prop shaft and mark a line from the brass (bronze) center piece of the prop to the outer shell (alum I guess) on the rear end of the prop. Put things back in place.

Run the prop in the normal fashion and after it does this again, stop, pull the stuff off and look at your marks. If they still line up let's talk some more. If not get another prop or a new hub in the one you have.

Mark

 

[tsp1158]

Re: Mercury Classic Fifty Cavitation??

Mark,
Bingo - you nailed it... Thanks a million!

I pulled the prop and I can see a bit of scoring on the mating faces of the prop hub and engine shaft.
The splines are perfect on both shaft & prop, but the prop has a ~ 2.5 inch "disc or plate" that mates to a similar piece on the engine. " A load or torque bearing surface??" Both have a little scoring like a car disc brake. Clearly I was (unfortunately) able to torque that slip hub more than I should have.
Good thing I only tested / tinkered for about 25 minutes yesterday & today.

I'm not too happy about that scoring. ... ever seen a shim to prevent this in the future?


Thanks Again,
Tom

 

[Texasmark]

Re: Mercury Classic Fifty Cavitation??

Great. The thing you are talking about is a thrust washer and that is what you force the prop hub against with the prop nut when you tighten it. You want the prop hub tight against that washer.

The inside diameter of the washer is tapered. the large (diameter) part of the taper goes forward so that the washer "wedges" onto the prop shaft when you tighten it up.

On my 90 hp engine the torque for that nut is 55 ft-lbs for the regular rubber slip hub. I'm turning a 13" wheel and you are probably turning a 10. So your shaft dia is probably smaller and your resultant torque is probably more like 45. I wouldn't guess. Call your Merc dealer and ask.

Glad you made it.

Mark

 

[tsp1158]

Re: Mercury Classic Fifty Cavitation??

Mark,

I just looked and saw the thrust washer and how easily it can be removed. It's scored, so I'll replace it.
Again - Thanks.
Will verify that torque on the shaft nut too.

BTW when I mentioned torque earlier I meant as in spinning the prop too much !

-Tom

 

[Texasmark]

Re: Mercury Classic Fifty Cavitation??

Great!