mercury 20 hp over revving problem

wylie catter

Seaman
Joined
Nov 16, 2010
Messages
61
I have a 78 mercury model 200 20 hp. Today I was coming across the lake almost wide open and it suddenly revved freely and quit pulling until I let off the throttle. Then it was good again. The free revving only lasted a until I twisted the throttle down. It did this every time I tried wot on the way in. It would run a lower speed with no problems.

Is this a prop problem of some kind.

Thanks, Terry
 

Mi duckdown

Commander
Joined
Apr 14, 2007
Messages
2,575
Re: mercury 20 hp over revving problem

It could very possibly the prop hub.
But If my memory serves well, Merc had a problem with18/20/25 in that year/s the drive shaft into the lower unit. It had a brass bushing instead of bearings..which it would slip into neutral.???
I went though that with Mercury.
I may be wrong. but not to far Off My rocker
 

Monte1961

Lieutenant Junior Grade
Joined
May 8, 2011
Messages
1,180
Re: mercury 20 hp over revving problem

Put engine in gear without running. Try as hard as you can to spin prop one way or the other. If it's the hub, you will know. Hint: just by hand only!
 

Mi duckdown

Commander
Joined
Apr 14, 2007
Messages
2,575
Re: mercury 20 hp over revving problem

If it where I. to check for hub slippage. I would mark the prop and the rubber hub. INDEX
run and see if those marks are slill lined UP after a test run. As long as you don't hit somethingthe prop is good. .02
 

wylie catter

Seaman
Joined
Nov 16, 2010
Messages
61
Re: mercury 20 hp over revving problem

Thanks for the info guys. I pulled the prop and it looks melted and like it has definetly spun. I'm kinda releaved. A prop is easy to change. Can anyone recommend a cheap prop website.
 

Mi duckdown

Commander
Joined
Apr 14, 2007
Messages
2,575
Re: mercury 20 hp over revving problem

Get it rebuilt . 35 bucks.?? Actually 70 bucks with hub and prop repair.
Michigan Wheel. Try there web site.
 

wylie catter

Seaman
Joined
Nov 16, 2010
Messages
61
Re: mercury 20 hp over revving problem

I've done a little shopping around and prices range from 89 to 130 for a new one. Mine has a 2 blade. I have found 2 blade and 3 blade props with exactly the same pitch and diameter. My question is, if the pitch is the same then wouldn't a 3 blade theoredically move more water?

Oh, and I do appreciate all the fast replies fellas. Yall make this an awesome site.
 

Mi duckdown

Commander
Joined
Apr 14, 2007
Messages
2,575
Re: mercury 20 hp over revving problem

If your motor runs fine with a 2 blade. don't mess with success.
Although most of them came with 3 blades.
Michigan knows what it came on factory. call them.
 

wylie catter

Seaman
Joined
Nov 16, 2010
Messages
61
Re: mercury 20 hp over revving problem

It runs good except it tops out before the rpms do. It'll push that boat 19mph at about 3/4 throttle. And going on to wide open doesn't gain any speed. It just raises the rpms on up. I had another of these motors a few years back and it had a 3 blade. It had a few more miles per hour on top end on this same boat. But it might've just been a little stronger motor. I'm thinking about getting the 3 blade. I can get it delivered and all for 119. And then I can still get my 2 blade rebuilt somewhere.
 
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