1975 Mercury 850 Thunderbolt 85hp

possom813

Seaman
Joined
Jun 22, 2010
Messages
67
I finally got into the motor on the rebel I picked up a few weeks ago.

It was rebuilt in 98 and the boat was parked in 2004-05 and hasn't been on the water since.

I put in new plugs, they're funny plugs that I've never seen before, no gap. Weird:confused:

Anyways, before the new plugs went in, compression check. All four cylinders are kicking at 90psi. I was thinking it should be higher, but I don't know.

I've got spark, got air, so I reckon all I'm needing now is fuel(and a water pump).

I don't know anything about the carburetors, so I'm going to leave them and the water pump to the boat shop, just as soon as I can get it in.

The wiring all looks good, except for one of the large gauge wires going to the starter, it was exposed outside and about 4 inches of insulation came off as I moved it around. The rest of it is in good shape, this was just a small area that was exposed. I went over it with liquid electrical tape with a couple of coats and then taped over it with self-vulcanizing tape.

I'm assuming it's not going to need much more to run. There is a short hose that appears to go from carb to carb that is weather cracked.

I'm wanting to get it up and running well and dependable, the shop I'm taking it to has always done good work for friends and they all recommended it. Is there anything specific I need to tell him other than make it run like a new motor?

And is 90psi about right, or do I need to forget the shop and find a new engine?
 

Wingedwheel

Lieutenant Junior Grade
Joined
Jun 16, 2009
Messages
1,071
Re: 1975 Mercury 850 Thunderbolt 85hp

Hard to believe that after a rebuild and low hours its only got 90psi unless your gauge is off. As long as they're all close its not bad. I have the same motor and other than POM,or neglect of I have had good luck with it. The carbs were really easy to rebuild and the water-pump is a breeze also. Unless you just don't like to mess with those things. You could save yourself alot of money by doing it yourself.
 

possom813

Seaman
Joined
Jun 22, 2010
Messages
67
Re: 1975 Mercury 850 Thunderbolt 85hp

I'd give it a shot, but I really don't have any idea what I'm doing. I can't find a manual on it and I don't know a lot about boat motors.

It doesn't seem like it can be too difficult, I just don't have any idea where to start on the impeller, and I don't want to screw it up too bad that the shop couldn't fix it after the fact.
 

dwco5051

Commander
Joined
Sep 14, 2008
Messages
2,430
Re: 1975 Mercury 850 Thunderbolt 85hp

Nothing really hard about replacing the impeller on that motor. Just remember to remove the trim tab to get at the hidden bolt underneath it
 

Wingedwheel

Lieutenant Junior Grade
Joined
Jun 16, 2009
Messages
1,071
Re: 1975 Mercury 850 Thunderbolt 85hp

The very first post on this forum will direct you to free manuals
 
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