TOPIC: An interesting troubleshoot on a '78 inline-6

The complaint was that the beast would run just fine, cruising or at an idle for about 20 minutes, at which point it would start misbehaving. reportedly like it was running out of gas, or cross-firing, or??? Then it would quit.
It would start right back up, but still not running right, and would run for only a short period of time before it would quit again, and again.
I found that if you waited longer after a failure, it was more likely run a bit longer, before quitting again. (from about 30 seconds to maybe a minute)
24 hours later, I found that the beast would start right back up, sound happy as could be, and run flawlessly again for about twenty minutes, before the aforementioned behavior reared its evil head again.
When i found there was no recovery with a shot or two of carburetor cleaner, or continuously squeezing the prime bulb, and a diaphragm kit for the fuel pump to eliminate potential flooding via a ruptured diaphragm...I focused on the ignition.
Surely it must be an overheated or cracked coil. In '78 the inline-6 has the smaller black coil, rather than the earlier - seemingly bullet-proof - large red coil.
I hung a known "good" big red coil on some zip-ties, and connected to the distributor (trigger), and original Mercury switch-box.
The following lake test went smoothly,...Right up to the 20 minute mark,...Then misfire - misfire - and she quit. Only to start right back up, but begin the failure sequences again.
So-o-o-o,...Since I have had some pretty crazy misfiring issues in the past that were caused by a failing trigger,...I swapped the trigger (distributor) out.
20 minutes later,...same failure sequence.
Only one part left, one might think - so I swapped out the switch-box.
20 minutes later,...Failure.
Now,...Besides shop time, I was burning a LOT of 92 clear 50:1 with AMSoil, through this guy's cantankerous old Tower of Power, and it is getting REALLY cold out on the water in the rain & wind driving in 2 mile circles.
Since this ignition is dependent on the stator and rectifier supplying it with proper voltage & a proper marine cranking battery, I checked voltage output to the battery. It was 17.4 vdc.
I've seen that high of voltage on the unregulated charging systems original to the inline-6, but had always encouraged folks to replace the unregulated rectifier with a later Merc regulator with a built-in rectifier, to control voltage by limiting output to either 13.8 or 14.2 vdc.
I've sold this update to folks with the explanation that the unregulated rectifier, when used with many different versions of today's batteries, can cause voltage spikes that can harm this style ignition.
This battery was a 400 cca marine cranking battery from 2009.
I shut down the beast and checked battery voltage. It was 13.4 vdc.
Obviously there was way too much voltage being thrown at this battery & ignition, and I suspected that must be why the ignition went crazy after about 20 minutes.
I installed a common seven wire Mercury regulator, and bundled up for another lake test.
No more failures!!! Wahoo!!
The owner called me a week later and reported all was well with the Mercury, and that there had been plenty of Salmon waiting for his return!!
NOW,...What I still don't know is: Will the original ignition parts that misfired and pooped out be okay now,...Or will they poop out again?? I suspect they will work okay, since the second, and third components I swapped out are still installed and doing well.
doc

I would be willing to bet ' that if you ran some lights radio depth finder ETC' it would have run fine.

I agree that accessories can help. I did an experiment once with a '79 140 that would build up to 17.5 vdc.
When it reached 17.5, I would run the CMC PT-130 up-and-down, and watch the voltage drop off for a bit - only to climb right back up in a minute or so. Constant additional load, like depth finders, lights, radios, etc would probably have done the trick, but who wants to NEED to run all their accessories all the time.
The regulator is a very simple fix IMHO.
doc

I simple HATE electrical crap! End of rant!