Re: Merc In-Line 3-cyl Issues
Yes, the little Three is a pretty highly-stressed motor, and there are design flaws in the cooling flow thru the block that Mercury attempted to remedy by relocating the telltale outlet to the top of the block, so it would be vented at the top and flooded with water (eliminating any potential 'steam pockets' which could overheat #1 cyl).
Also, the impeller is the same size as a 50 HP so there's no margin for error. Most folks won't replace impeller every season and that's about what you have to do to keep these alive.
In addition, the small displacement motor doesn't tolerate overpropping or overloading on a larger boat; it's best suited for light loads on a small boat.
That being said, the 4-cylinder model, although having the same bore/stroke as the 3-cyl, does not have near the problems the 3-cyl had.
Just make sure you can see the 850 run, preferably on a boat in the water. Check idle quality carefully as this is a good indicator of the condition of the innards. Take a compression check at minimum and if you can't run it on a boat, at least run on a flusher to see if it's making any bad noises or has other issues.
Note on the compression check, if any cyl is more than 5 psi off from another, it's indicative of potential issues. Preferably they'd be all the same but hard to get that in a 34 year-old motor!
Check the gear oil by pulling the lower (drain) plug. The gear oil should be clean, not milky-colored and no gross amounts of metal flakes when you rub a drop between your fingers. Signs of lots of water may indicate driveshaft corrosion problems which can be very $$ to fix.
Anyway, HTH and G'luck on your prospective purchase...........ed
