Where to start?

[Jb3]

New here, just picked up a project boat 74 astroglass with mercury 850 outboard, boat was free this could be good or bad stuff never looks as bad in pictures until u start tearing it apart lol, this boat allegedly floated and ran at one point but has been sitting outside in the elements for an extended amount of time, wiring is shot on boat, my concern is the motor and where to start, I am an auto technician but have no experience with outboards or even carbureted engines for that matter, so aside from checking compression I'm just looking for a guide to assess damages as far as common issues or maintenance items related to this outboard is concerned and what to look for , also much of external engine components look to need attention as well ( cables, wiring, etc) any guidance would be appreciated.....also on a side note looking for engine identification only visual serial number found on inside of transom mercury bracket is #4233793 having trouble finding any identification for that for when I need to order parts, thanks for any help

 

[roscoe]

85hp

850

4-cyl

4092965 -

4366801

1975

USA

compression spark fuel

don't mess with timing, outboard timing is not like an auto timing.


Get a manual, preferably factory hp and model specific, if you can't find one, buy one of these:
Clymer B726 1972-1989 Mercury 45-225hp Outboard Maintenance Service Shop Manual



Compression test with all plugs removed and throttle wide open, spray a little light machine oil or 2 cycle oil in cylinders first

rebuild carbs - use Mercury factory carb kits
Change water pump impeller
change gearcase lube
clean and tighten all electrical connections, including grounds, one at a time.

 

[Jb3]

Thanks for the advice roscoe!!

 

[DeepCMark58A]

rebuild fuel pump and check valves in the fuel pump, use only mercury parts as well.

 

[Mercurylips]

Can't say enough about water pump rebuild. It can't be good after this long. If you start it without the water pump rebuild, don't run it long. Good luck.

 

[dfrants]

another possible issue is the control box. The wiring inside is probably shot. I have a dozen of those style and one is nice, one I rewired and the rest are junk for parts. Also be careful under the cowl removing parts with wires. They are most likely dry, corroded and brittle. A major pain to replace...believe me, ive been there.

 

[Jb3]

Thanks for the input guys, I plan on doing a compression test and then proceeding with rebuilding all the engine components u have all mentioned, I would rather go thru everything now while it's coming apart rather than later, i plan on rewiring everything since the insulation is coming off everything or very brittle if it's not, so I will check the control box wiring as u mentioned as well. I'm sure I will have more questions as I run into unforeseen problems or lack of outboard experience on my part I appreciate the help

 

[Chris1956]

OK, a couple of things you may not know. A 1975 Merc 850 will have a battery powered CDI ignition system with a distributor. The cap and rotor are very expensive and nearly never need replacing. The rotor is non-removeable and fragile. Check the carbon electrode in the distributor cap. Check the spark plug wires for cracks and replace if they arc to ground. The starter needs to have a dedicated ground cable, as it is mounted in rubber. The engine wiring harness is usually bad by now. They are available at several places on the web, so shop for price.

The motor is an inline 4 and has no thermostat. As the engine speed increases, the waterpump pumps more water to keep her cool. If she is overloaded, or the waterpump is damaged, she can overheat easily and cause much $$ damage. The carbs are simple to clean and rebuild, as is the fuel pump(s). The top of the carbs likely have banjo filters, under the caps. The two carbs have individual idle mixture settings, however, they are not that hard to adjust. Final adjustment must be made on the water, in gear, motor warm. More when you get that far.

Link and synch refers to setting the carb to open at the proper ignition timing. That is critical as is max spark advance,

All in all the motor is a good one, IMO. Their idle is not the best, but they have good power, in a compact lightweight package. Parts and props are widely available.