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Author Topic:   Over Revving?
MilwaukeeWhaler posted 05-21-2002 12:00 PM ET (US)   Profile for MilwaukeeWhaler   Send Email to MilwaukeeWhaler  
After an evening of trolling with my '95 90 Merc at idle speeds, my motor, when at WOT, was over revving and barely on plain. I finally ran thru the winter gas and had just filled with 27 gallons of 93 octane. Any thoughts? Can I ad carb cleaner to my gas? Prop looked fine, plenty of oil. Haven't checked the plugs yet. I am heading out tonight. What should I look for? Thanks in advance for your help.~mw
Whalerdan posted 05-21-2002 12:10 PM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
Props got to be spun or the gears in the lower unit completely gone. Can't be anything in the power head. Oh yah, just my opinions.
whalerron posted 05-21-2002 12:16 PM ET (US)     Profile for whalerron  Send Email to whalerron     
I concur. It sounds to me like the rubber piece in the prop hub has deteriorated to the point that it is slipping and it is letting the propellor shaft spin faster than the prop.
lhg posted 05-21-2002 12:29 PM ET (US)     Profile for lhg    
I'd bet on the prop also. I had a rubber hub "spin out" on me also. Costs about $60 for a repair. Mercury's new interchangeable plastic hub props don't do this anymore.
lhg posted 05-21-2002 12:32 PM ET (US)     Profile for lhg    
Further to above, a "spun hub" prop can be difficult to remove, since you can't block the blades to unbolt it. They had to wedge the engine flywheel, in gear, to get mine off.
MilwaukeeWhaler posted 05-21-2002 01:12 PM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
Gosh I hope not. I wasn't seriously over reving. I was 6000 rpm at full throttle. I usually am at 5500m. Speed was 31 instead of 41. I thought the motor was shaking a little more than the usual Merc shake. It also seemed to bog out when opened up. I still think it may be a carb problem.
Clark Roberts posted 05-21-2002 05:04 PM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
I concur with the excellent advice above! Check out the prop first! Happy Whalin... Clark...SCN
David Ratusnik posted 05-21-2002 06:14 PM ET (US)     Profile for David Ratusnik  Send Email to David Ratusnik     
Ran into a guy the other day with a nice mid 90's Conquest. Asked how he liked the engine. Said the engine was fine but those Merc prop hubs "are crap." Last 2 weeks he had hit a wave hard and spung the prop limping home. Milwaukee- agree the over rev is spun prop. Anecdotal--David
Tom W Clark posted 05-22-2002 02:05 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

I'm with these guys. Most likely explanation is a spun prop. The evidence you have provided rules out a carburetor problem or any other flaw in the powerhead resulting in reduced power and boat speed.

If your rpm’s are higher than normal but your boat speed is lower then there must be a problem with the connection between the powerhead and the prop. A spun prop accounts for this.

There is one other explanation that comes to mind though: Ventilation.

If there is something on the bottom of your hull (a new transducer?), or the mounting of your motor has changed significantly (motor raised several holes or trim and tilt way off?) then your prop could be sucking air and allowing the engine to achieve greater than normal rpm’s while simultaneously resulting in a slower top speed.

What have you changed over the winter?

whalerron posted 05-22-2002 08:23 AM ET (US)     Profile for whalerron  Send Email to whalerron     
I have always been a "Johnson-guy" but I cannot ignore the awesome power of a Merc. I don't think that the Merc props are an issue. The real issue could be that the Mercs have such tremendous low-end that the props can't handle the power. You gotta respect a motor with that much power!!
MilwaukeeWhaler posted 05-22-2002 10:11 AM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
Thanks for the good advice! I have not changed anything over the winter. I have had the boat out 10 times this year and it has run great. The only thing I noticed early in the year is that my idle speed increased one day by aprox one mile an hour. Ok now that we have this narrowed down, will it be obvious when I try to take the prop off that the hub is bad? How difficult is it to change the hub? Sounds like the tough part maybe taking the prop off. I am taking my boss out tomorrow and need a quick fix. Would you take the boat out with a bum hub? I don't have to go far. Call me crazy!~mw
MilwaukeeWhaler posted 05-22-2002 10:15 AM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
I assume the hub is part of the prop not the shaft. I have a spare prop. Changing props should alleviate the problem right? Stupid questions I know. Thanks
Tom W Clark posted 05-22-2002 11:12 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

The hub is part of the prop. You may not necessarily be able to tell with your hands if the prop is "spun".

Try the spare prop. Assuming it is the same and undamaged, this will tell you if a spun prop was your problem.

If it is, take the spun prop to a prop shop and they will replace the hub for around $50.

MilwaukeeWhaler posted 05-22-2002 11:21 AM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
Great. Thanks Tom. I will keep you posted.
MilwaukeeWhaler posted 05-24-2002 09:06 AM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
Well I pulled the prop last night and saw nothing indicating a spun hub. (I called Merc and they said there should be plastic or metal shavings on the shaft.) Put it back on and launched with no sign of any problems. I am begining to think it may have been a clogged intake or something. I am cautiously optimistic!~mw
Dr T posted 05-24-2002 10:40 AM ET (US)     Profile for Dr T  Send Email to Dr T     
Just a thought (and based on my ignorance): Could this behaviour be an early warning that the prop about to "get spun" and by re-installing it you have tightened it down so that the friction of the prop nut, not the splines, are holding it in place?

If this is the case, it could get bad in a hurry the next time it hits an unusual amount of stress.


Tom W Clark posted 05-25-2002 01:21 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

Reading your descriptions I am thinking you probably just had some debris hanging on your gearcase which can cause ventilation with a resulting loss of speed and over revving just as you have described. I suspect you have no problem at all.

Dr T, I don't think so. Tightneing the prop nut shouldn't affect the grip of the hub one way or the other.

JLL posted 05-25-2002 08:32 PM ET (US)     Profile for JLL  Send Email to JLL     
From an old Mercury Site
Best way to determine if hub is spun.
File a mark on end of prop shaft. On every prop to be tested, file a corresponding small notch or scratch on the surface of prop shaft so that any change in their lineup will be evident. Run motor hard, stop & recheck mark lineups. If they no longer line possess a bad prop with a spun hub.

triblet posted 05-26-2002 09:12 AM ET (US)     Profile for triblet  Send Email to triblet     
That would test whether the prop splines were
spinning relative to the shaft. That's not
what happens on a spun hub -- the prop
spins relative to the prop splines. So mark
the prop body and the prop splines.

And try this. Remove spark plug wires.
Put motor in gear. Try turning prop. If
it's badly spun, you'll be able to tell,
though from your description, I don't think
it's BADLY spun.


jimh posted 05-26-2002 02:04 PM ET (US)     Profile for jimh  Send Email to jimh     
Generally a prop will have a splined thrust washer between the prop and the engine. If the prop hub has spun, there will be circular scratches on the prop or thrust washer which occured when the two rotated out of sync. Look for these scratches as evidence of a hub that has spun.
MilwaukeeWhaler posted 05-29-2002 12:15 AM ET (US)     Profile for MilwaukeeWhaler  Send Email to MilwaukeeWhaler     
Glad I asked. I learned a lot in this thread. Honostly, this sight is incredible. The answers I got here were of far greater value than those I received from Merc or my dealer. It is amazing the wealth of knowledge we have assembled here. (I say this running the risk of sounding like a brown-noser however we all know its true) I truly feel this sight has made me a safer and happier boat owner, Thanks again.~mw

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