1970-1973 mercury 850 thunderbolt

I have a mercury 85 horse thuderbolt that will not start. When I bought it, it didnt have the internal harness, so I bought that. I wired it up, then it had no spark, so I put new plugs in, still no spark bought a cdi box, put that in. I have spark now, but it wont fire. I took the carbs off to see if they were dirty or the float was bad. Nope they are fine, but got a good cleaning. Ok, so I adjusted the throttle screws to make sure the carbs were opening and stuff, and it does. I have good compression about 115 apiece give or take 1. I have spark on three of the cylinders. I cannot figure out what the problem is. The distributer is good, the wires are good, the plugs are great, I have gas, and compression, the question stands why is it not starting. Do any of you have any ideas. I also looked at the timing belt and that is fine.

Re: 1970-1973 mercury 850 thunderbolt

Howdy, dlaruffa.

Welcome to iboats.

Go to the Engine FAQs and do "Outboard won't start". Let us know what you find.

Good luck.

Re: 1970-1973 mercury 850 thunderbolt

On that dissy motor, make sure you have +12V on the white wire on the switchbox when the ignition key is on. The red wire should have +12V all the time. If one plug doesn't have spark, it is likely a bad plug wire or a carbon track in the dissy cap. Clean the cap with some solvent. The rotor is non removeable, so be careful with it. The cap is $100 to replace. The plug wires may be found for $6 ea at Vintage outboards.com. You can replace the orig rectifier with a radio shack 25 A full wave bridge rectifier.

Re: 1970-1973 mercury 850 thunderbolt

I will check the voltage. Thanks.
Do you know where I can get teh distributor cap, and or the whole assembly.

Re: 1970-1973 mercury 850 thunderbolt

dlaruffa said:

the question stands why is it not starting. Do any of you have any ideas.

Click to expand...

Don't forget to check the kill switch....Happens all the time.

Re: 1970-1973 mercury 850 thunderbolt

Those old ones have the kill switch?
I am wondering if any of you know if the 63-64 85hp parts will fit on that one.

Re: 1970-1973 mercury 850 thunderbolt

The kill switch on those motors is a mercury switch which grounds the switchbox, if the motor is tilted too far up. they are blue and mounted on teh front cowling support.