1975 merc 850 spark test help

[ronward]

I have this old 85 horse I wanna try to get running. I am more into OMC's so I'm not sure where to start on this merc but it was given to me so...here I go. Compression is good at 140-145 on all four cylinders. I tested the spark as I would on an OMC and got nothing. Maybe you do it differently on a Mercury. How are you supposed to check spark on these. I used the same tester as on my other motors. Is there something else I need to do? If I did it right and have no spark what do I do next? What order would you guys check things?

 

[MH Hawker]

Re: 1975 merc 850 spark test help

Should be a standard spark gap tester. And if no spark then I would do the ohm tests on the stator and trigger, and on some you can also do a ohm test on the switch box.

 

[ronward]

Re: 1975 merc 850 spark test help

I went to the CDI site and it mentions a mercury tilt switch connected to the switch box and that it could cause a no spark if it's bad. It says to d/c it and re-test. Would this motor have one? Ser no 4298192. I have a cheap analog multimeter. Could I do the ohms tests with it or need something else? How would I do those tests? I've been looking at electrical parts prices for this motor...after I picked myself up off the floor I realized this might make a better anchor than a motor

 

[Chris1956]

Re: 1975 merc 850 spark test help

The mercury switch should be on the center cowl support on the port side. It will be a small blue cylinder. Looks like a condenser from a points/coil ignition.

You have a distributor on that motor? What shape is the engine wiring harness?

 

[eggs712]

Re: 1975 merc 850 spark test help

Make sure you're not just jumping the starter to test for spark. The ignition key has to be on to get spark.

 

[MH Hawker]

Re: 1975 merc 850 spark test help

and yes depending on how the tilt sensor is positioned yes it will kill the spark , its should if the motor hits some thing while your running and it gets kicked up , for testing you can simply disconnect it. Its open in the normal running position and it closes if the motor is tilted up to far.

 

[ronward]

Re: 1975 merc 850 spark test help

I did d/c the mercury switch. Hooked everything up right, still no spark. Tells me...switch box. The motor is already gone from here. I wont pay what they want to fix...junk. Sorry guys, thanks for the help but I'm stickin with OMC

 

[eggs712]

Re: 1975 merc 850 spark test help

You can find some used ones on eBay for only $100. I have a motor I'm parting out right now with the same switch box you need. PM me and I can make you a deal.