8N quits after 3 rounds

What others suggest, but you didn't mention checking the points, condensor, etc. Further, an improper 12 volt conversion could cause your coil - new or old- to become to hot. When it colls down it'll start up and run until it gets hot again.

You might consider checking all of the three (3) fuel filters, 1) filter at fuel shut-off valve, 2). at the carb, inlet, and 3) the screen located inside the glass bowl assembly.

There is also a fuel tank "vent" which, if clogged, will stop the flow of fuel. To check the vent, sometime try running the tractor with the fuel cap loose. If the vent is clogged, it will be necessary to remove the tank.

Hope this helps. Others will probably have more ideas. :})