73-74 mercury outboard 850 thunderbolt 85 hp. No Spark

[mattie24]

Motor was running good. Then one day I could not get it to start. I have no spark on all of the plugs. I found some old wires on the internal harness and replaced that, with the same results. Distributor has no cracks or burns. replaced the carbon tip also. Any help would be appreciated.

 

[thunderbolt1977]

Re: 73-74 mercury outboard 850 thunderbolt 85 hp. No Spark

did you check your switch box? did you test the spark plug wires to make sure that they are getting a spark to them? Did you replace all of your plugs? is your battery fully charged? I have been having spark problems with my motor for a long time and just solved it just some ideas for you to look at.

 

[jeffreyr]

Re: 73-74 mercury outboard 850 thunderbolt 85 hp. No Spark

Along with all the stuff he just said, the stator and trigger are two other things to check..

 

[mmf]

Re: 73-74 mercury outboard 850 thunderbolt 85 hp. No Spark

disconnect the mercury switch and also remove the ignition stop wire from the switchbox (grounded when in stop position on ignition switch). If you now have fire, you have a bad switch or a grounded wiring harness. The wiring in these older mercury's are notoriously rotten. You can still get a new internal replacement harness from boats.net or cdielectronics.com, the most overlooked wiring problems I see if the internal wiring is rotten is at the ignition switch where you will also find wire rot. I have cut the wires there on many of them and cut the harness back to good wire and resoldered the ignition switch with success.

 

[Chris1956]

Re: 73-74 mercury outboard 850 thunderbolt 85 hp. No Spark

Mattie, That motor needs +12VDC on both the non-distributor white wire and the red wire on the switchbox. If you have this voltage, get on the CDIElectronics.com website and run the troubleshooting test for the Battery powered CDI ignition. It will tell you if the wiring and switchbox are good. The stator plays no part in this style of ign, so it is best ignored. Disconnect the blue mercury switch as well.