1977 Mercury 20HP mystery ignition problem

[Mr. Awesome]

HELP!!! I bought a mint “looking” 1976 or 77’ Mercury 200 (20hp) about a month ago. Serial # 4354347. The guy I bought it from said he barley used it and I found out why the hard way. First time I took it out, it missed at mid to hi rpm range then would die completely after 15 minutes. No spark at all. After about an hour and a half, it would start up again. It would idle fine but miss at mid to hi rpm range.
I bought a Mercury dealer recommended repair manual and learned everything the book had to offer about my type IV ignition system. Stator coil, trigger and ignition coils tested out perfect. I purchased a new aftermarket switch box from CDI and installed it. I also rebuilt the carburetor.
Ran the motor in a test tank for a few minutes and was impressed with the smoothness of the idle. Next I took it to the river. Put the boat in and off I went. Motor ran great! For 15 minutes. Motor idled fine then kicked it up to ½ throttle, ran fine. Finally ¾ throttle for about 4 minutes then just like you hit the kill switch, it died. No spark at all. Not even after an hour or two.
Took it home and tore it down again, all the parts tested fine. Found a complete power head with electronics locally for $50. Took the switch box off the $50 power head and replaced the brand new switch box I had just installed on my motor. It was already fried! Put my motor back together with its original tested out parts and the switch box off of the $50 power head. Success! Or so I thought.
Again it ran fine in the tank but did the same thing out in the water. Ran great for about 7 minutes at ¾ throttle then died just like you hit the kill switch. Again I have no spark and the stator, trigger and ignition coils test fine. I’m plumb out of switch boxes and ideas. Every Mercury mechanic tells me the same thing. “ I don’t know ”!
I inspected all the grounds and the kill switch. It all looks like it just came out of the factory. Any ideas?

[7920hpMercury]

Bummer indeed!

These are normally great motors.

Just guessing, but sounds like with new parts and when it is cold, it runs great. When it gets hot or up to operating temp, the ignition quits working.

I know the original wires were fine aluminum wires and they corrode out in some strange ways and some strange places.

These kinds of intermittant problems drive people nuts. But I think I would swap out every piece of the ignition and wires with the parts from the 50$ motor and clean all of the grounds and continuity test all of your wires.

Hope this helps and sorry I can not offer more.

[Mr. Awesome]

Thank you for the tip! I'll do that this weekend. My new switch box should be here by Friday. Do you think my two switch boxes are toast since I didn't have spark with them after the motor died? Is there a way I could test them? The manual dosen't say anything on how to test out switch boxes. Is it possible the stator could be producing more voltage than the switch box can handle? Any and all information is greatly appreciated! Thanks again!!!

[7920hpMercury]

You need a DVA meter to check the switchbox. Do a search here and check Laddies's posts and/or CDI home page/Technical help for DVA values. If you can not get it to spark, and all of the other items ohm check good, I usually deduce it is the switchbox. If you are burning up switchboxes, something is getting hot and doing the damage is my guess.

Hope this helps.