WOW...bpperk, you love your yellow iron as much as I do...and thanks to TX JIM, I am now very sure my faithful JD300 is a closed system.
The machine I bought last year runs but has some issues...which I shall go over at another time. But for now, what I had to do was put in a 2.2 ohm resistor in the wire to the coil because the wire apparently lost it ability to resist and the coil was throwing high voltage back into the battery. The multimeter showed over 20 volts spiking. I thought about this for a long time and finally tried an experiment. I took a piece of old heater coil wire from a burned out Maytag dryer and cut it to a length (approx 12") that made 2.2 ohms and cut the wire going to the coil and using clips, put the resistance wire in the circuit...voila, no more spikes and I'm sure the coil will run cooler and the points and condenser are probably liking it too. Hope that helps someone. Those old JD300 are nice...small but not too small...cute...when I start it, I destroke the pump with the little destroker valve and the engine starts so much easier.