Re: merc 850 no fire
A 1976 850 has "type 8" ignition with (4) coils, one switchbox, no distributor. Is this what yours has?
If so, there are a few things you can do to troubleshoot. Disconnect the (2) stator (yellow) leads from the rectifier and see if that makes any diff. If so, replace the rectifier.
Disconnect the kill wire from the switchbox. Should be a black wire with yellow or white stripe. Some older models had an orange wire, you can tell this is the kill wire because it'll also be tied into the "mercury switch" which shorts out to kill the motor if it's kicked up by an underwater obstruction while underway.
If you get spark with the kill wire disconnected, also check for voltage on the wire with the ign switch in the "ON" position. Voltage on this wire will ruin a switchbox and is most likely caused by a bad ign switch. This wire grounds the switchbox with the ign "OFF" and so should be a Dead Short with the key "OFF" and infinite resistance (or extremly high resistance) with the key in the "ON" or "START" position.
Disconnect the Hi-Speed and Low-Speed stator windings from the switchbox and do a resistance check.
Disconnect the trigger wires from the switchbox and do a resistance check.
If all the above appears normal, most likely is a bad switchbox. Highly unlikely that all four coils would fail simultaneously.
You'll need an analog or digital meter for the resistance checks, Sears is a great place to find a meter on sale. Even an inexpensive one should be up to the task.
Here's some troubleshooting info and resistance specs:
http://www.boatpartstore.com/mercreadings.asp
http://dolphinmarineservice.homestead.com/ign5.html
Here's a wiring diagram for a 4-cyl ADI motor:
http://www.maxrules.com/oldmercs/Wiring/1966ona/80_85_5582561_BACK.gif
Anyway, that should get you started. Recommend a service manual for any advanced troubleshooting. HTH & G'luck........ed
