Re: Merc 850 timing
This is true, the factory did come out with a recommendation to back the timing off a few degrees. 24 deg BTDC oughta do the trick.
BTW if you have "back-drag" style carbs with a brass-colored 'vent jet' at the top of the float bowl, you should remove the jets from both carbs. And plug with a BB the little black hose that runs from the carb throat to the float bowl cover.
Back Drag carbs were designed to lean out the mixture at mid-range for better economy, with today's fuels that's a recipe for burnt pistons!
Check here for a writeup on the Link 'n Synch procedure:
http://forums.iboats.com/showthread.php?t=168855
Note the 21 deg BTDC spec is for Inline Sixes, your I-4 is good at the higher 24 deg spec.
HTH.........ed