Probably not worth saving? '74 Merc 850 85hp 4-cyl

[Fastertelecaster615]

Hey folks
I am totally new to boating, however, I have rebuilt 4-stroke vintage Honda motorcycle engines, so I am not afraid to dive into a marine engine.
I got a 1974 Fabuglas Fishing Boat (14.6") with a '74 Merc 850. I got it with a BMC trailer for a song, so I am ok if someone says "send it to the dump and start over"
It was advertised as needing a new wiring harness. There were two automotive type cables coming out of the cowl hole (lower right) that went to the starter, and the starter solenoid. It appears complete (nothing seems to be missing). The starter motor worked so I thought I would test her.

I rotated the prop and all seems well. I pulled the plugs and did a compression check on each cyl and got 135/140/135/125 (1,2,3,4)
I downloaded a manual from recrepair.com but it is a generic "how to service 1965-1989 Mercury Outboards"
There are lots of specs in the appendix except for expected compression. Anyone have this figure?

I suspect I may have sticky rings in cyl 4 but maybe all are bad if compression should be 160+.
Looking for a definite answer on the spec compression figure. Thanks!

 

[GA_Boater]

Welcome aboard.

If it's been a while since the motor has run, I wouldn't worry about the compression numbers. Actually they are good - Within 10%.

Have you checked for spark? If you have spark, squirt some pre-mix in the plug holes and crank her over to see if she spits.

If you see she will fire. then the decision is about dropping some money for a wiring harness, impeller. carb kits, etc. iboats has all those in the store.

Good luck!

 

[frozenokie]

FYI - I rebuilt a 73 Mercury 1500 (115 hp) and was only able to use a WISECO piston which is a high performance piston... When I traded the boat and motor in on a new boat, they tested mine out first. The mechanic was amazed that he was getting readings of 150-150-150-150-150-150 on the nose!!! He mentioned that is a very rare case to even see a 150 reading let alone a consistent 150 across the board!!

I was told by that mechanic that most brand new engines have roughly 120-125 for each cylinder.

So your old Merc 850 should purr quite nicely when you get the fuel/carb thing figured out

 

[tpenfield]

Sounds like the motor is a keeper, what about the boat ?

 

[CV16]

I would change the impeller before trying to start it. Or at least take it out and use muffs to force water through. You sure don't want to have to find little pieces of rubber if the impeller galls apart on you. It's not fun, been there done that.

 

[clueless75]

I have a 1975 merc 850 and compression was all between 137 and 150 psi.You should be fine with those readings and she should fire right up!