Mercury 850

[kenmyfam]

Buddy of mine has a Mercury 850 he believes is a 1979. It was too difficult to get to in his garage this evening to get any numbers from. He has a 650 also which is giving no problems.....but he would like the 850 on his boat for obvious power and hole shot related issues. He has a 16 ft runabout type boat.
Problem is that with the 850 on the back it wil go really well for about 5 to 6 minutes and then die on him. he lets it cool down for 5 minutes and it will follow the same procedure again and again. Just before it quits it will just dribble cooling water from the pee hole but for the rest of the time it appears to be cooling well.
I know absolutely nothing about mercury outboards (I run a Johnson 140hp looper) so I am asking on his behalf for advice before boating season starts here. He will be joining up here as soon as he gets his computer up and running again.
My thoughts are that he needs to replace the impeller and thermostats to begin with and go from there but that is just a guess from my standpoint. He gets no warning buzzers but there again i do not know if they have them or not or if they are connected.
Any advice would be greatly appreciated.
Thank You,
Ken.

 

[Plainsman]

Re: Mercury 850

Maybe switchbox? When it gets hot it malfunctions and loses spark? Someone else will be along to give you more advice.

 

[kenmyfam]

Re: Mercury 850

Have trouble understanding the cooling water slowing to a dribble though the rpm stays up there if it is the switchbox ???
Thanks for the info thus far.

 

[Plainsman]

Re: Mercury 850

I didn't see the part about the RPM's staying up, I thought it slowed the pee then died. So it stays running, but quits peeing?

Anyway, if it's a new to him motor, I would replace the impeller, who knows when it was last changed.

 

[emckelvy]

Re: Mercury 850

Is the engine block extremely hot after the motor quits? Feel the exhaust cover (Port side of block) and if's it's more than lukewarm after a hard run, there's not enough cooling water.

Make sure the motor is the correct length for the transom. The anti-ventilation plate (the large horizontal 'fin' around the upper part of the lower unit, more commonly - and incorrectly - known as the 'cavitation plate') should be even with the keel of the boat, or up to around 1" above the keel, depending on the hull. If the motor is mounted too high it'll ventilate (suck air and speed up) in turns, and maybe even lose cooling water.

A more likely suspect would be the impeller and as previously recommended, if you don't know when it was last changed, change it now regardless! Cheap insurance in any case.

As for the ign, you can carry a spare spark plug with you and when it quits, pull any spark plug wire, stick the spare plug in it and lay against the block. Crank the engine over and see if there's a fat, purplish-in-color spark.

BTW it would help to have the serial # of the motor as it'll help determine the exact year and type of ign it has. It'll either have a belt-driven distributor with (1) ign coil, or an ign box on the Stbd side of the motor with (4) ign coils.

Each type of ign has it's specific problem areas and we can't begin to troubleshoot without knowing which type you've got.

P.S. there's no thermostat on this motor, and the 'dribble' could be as simple as debris in the telltale. Install a new pump kit, and blow the telltale out with an air hose; I'll bet any cooling issues will be gone.

HTH..............ed

 

[kenmyfam]

Re: Mercury 850

Thanks for that. I will forward all info to him. Did not know there were no thermostats in this motor but it seems that everyone agrees on an impeller so far so that appears to be his first job along with a pump kit to install.
Thanks again for the info.
ken.

 

[kenmyfam]

Re: Mercury 850

I have sent the advice to my friend and he will be getting on it next weekend. Is there any other input ??

 

[Plainsman]

Re: Mercury 850

Quote Kenny: "Have trouble understanding the cooling water slowing to a dribble though the rpm stays up there if it is the switchbox"

I see nothing about RPMs staying high in the post, do they stay high or does it die? Big difference there. If it starts to just spit water and it dying, that might expain that. But if it keeps running and just spits, different senerio. Do we have a serail # yet? just my .02

 

[kenmyfam]

Re: Mercury 850

Apparently the cooling water slows to a dribble with the RPM still up there and then it dies very shortly after that.
Sorry I was not clear on that earlier. Still waiting serial number.
Thanks for your input and help.
Ken

 

[jimmbo]

Re: Mercury 850

Just for info... The 4 cylinder 850 was made 1973, thru 77. In 78 it became an 800

 

[kenmyfam]

Re: Mercury 850

Thanks Jimmbo, just another reason that I need him to get me the serial number !!!!!

 

[jimmbo]

Re: Mercury 850

Post a picture of it. At least we can tell you what year the wrap is

 

[kenmyfam]

Re: Mercury 850

Will get him to send me one. If he doesn't get me the numbers by the weekend I will go get them myself. By the way, where are they on a mercury ??? I only know Johnson / Evinrude.
Thank You

 

[jimmbo]

Re: Mercury 850

The boat in the avatar... Is that a Crestliner 550 crusader? Or a larson?

 

[kenmyfam]

Re: Mercury 850

It is actually a Doral 164 (1985) The brochure for that year is posted in the Doral section.