Question about 850 mercury power loss

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Black69

Seaman Apprentice
Joined
Jul 13, 2005
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49
I recently purchased a 76 Starcraft with Mercury 850. The boat ran fine prior to selling it to me, and after sitting a few months. My first trip to the lake was not good. The boat would idle fine but under power would not get up and go. Prior to purchasing the boat I found out that he had taken the prop off and forget to replace the thrust washer. There did not appear to be any damage. My question now is..if you put the shifter into forward gear should you be able to turn the prop freely or should there be some compression from the engine?
 
JasonJ

JasonJ

Rear Admiral
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4,163
Re: Question about 850 mercury power loss

If you put it in forward, and rotate the prop in its forward thrust direction, it should ratchet. If you try to turn it backwards, it should not turn. You would not be able to rotate the engine from the prop, and you would not want to do that. It sounds like you may need to go through the carbs, they may be dirty, fuel filter may be gummed or clogged. Fresh plugs, fresh gas, clean carbs, thats what you need most likely. Now, you said it ran fine before you bought it, is that based on you test running it on the water, or because he said "it ran great last time I used it"?
 
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Black69

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Re: Question about 850 mercury power loss

It ran fine by the last owners comments. He does happen to be my neighbor across the street however, so I hope he's being honest. I did replace the o-ring from the fuel line connection to the motor and it was way out of shape. Put a new one in and haven't had the chance to test run it yet. It almost acted like it was starved for fuel and almost like it was missing with no backfiring. Checked the plugs and they were fine. Is there a fuel filter on the motor rather than where the fuel line connects and the screen inside. I checked the fuel tanks and they were fine and it was new fuel. Any ideas would be helpful. Thanks in advance.
 
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Black69

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Jul 13, 2005
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Re: Question about 850 mercury power loss

For those of you with an old 850, does that o-ring have to be replaced much? The previous owner told me to always have a spare but I didn't think I would need to replace it very often. Any info would be helpful. Thanks.
 
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Black69

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Jul 13, 2005
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Re: Question about 850 mercury power loss

Anyone, I see that some 850's had a gasket and not an O-ring...Any help would be appreciated.
 
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laker925

Petty Officer 3rd Class
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Jun 10, 2005
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85
Re: Question about 850 mercury power loss

Mine had a gasket and no o ring...The kit had it in there but it was not on mine.
 
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Black69

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Joined
Jul 13, 2005
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Re: Question about 850 mercury power loss

It seems that the o-rings after some time expand and create a leak allowing air into the system. I'm wondering if the gasket material might be better?
 
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Black69

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Jul 13, 2005
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49
Re: Question about 850 mercury power loss

Any ideas on the gasket? I think I may have caused it by running some ethanol through it. Maybe not though, the previous owner said that he doesn't remember ever replacing or having a problem with the o-ring. Any opinions?
 
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donspaulding

Cadet
Joined
Jun 23, 2005
Messages
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Re: Question about 850 mercury power loss

Dave,<br /><br />I also have a '76 850 and had to replace that same O-ring when I started working on the motor this spring. The powerhead had a few nicks on it and the old rubber ring was letting gas leak out when it was primed. A little JB Weld and a new O-ring was all it took.<br /><br />What is your most recent concern/question?<br />Did you figure out your power loss?
 
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Black69

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Jul 13, 2005
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49
Re: Question about 850 mercury power loss

It seems tht the o-ring after a week or so begins to expand and then allows air to enter. If I idle around it's fine, but when I want some speed the motor seems to hesitate and lose power. When I check the o-ring it pops out and is way to large to fit back it. Originally it is the right size but after some time it seems to expand. That's way I was wondering about using gasket material to replace the o-ring.
 
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