Farmall M 12 volt neg gnd loses charge

I bought a 1940 Farmall model M (with magnito)at auction. This tractor had been nicely restored, painted, and converted to 12 volt negative ground including alternator and resistor.

My problem is that if this tractor sits for more than a few days without running (charging), the battery will discharge over time, requiring a battery recharge or jump start.

Wiring includes a push-pull mag kill switch, and an on/off toggle switch for exciting the alternator field.

Any thoughts?

Try using a "hold against" ammeter on wires going to the alternator to check for discharge. If no current is being drawn there, go to other wires to find the source of the discharge. All this with the engine off. Could be a battery with a shorted/ defective cell also.

If your alternator has an internal regulator it could be the problem. If you don't use the tractor very often disconnect the battery's ground
until you're going to use it again. Hal

As simple as your system is, I would pull the alternator & take it in for a free test at loval auto supply. If you wan to narrow it down a bit more before removal, then the suggestions of current 'snooping' or looking for small sparks when connecting/disconnecting alternators wires are good. Or you could remove connections to alternator next time you park it and see if the dead battery still occurs.

With a on off switch to the alternator "excite" terminal, (assuming you shut that switch off when the tractor is not running) the tractor seems to be wired OK. (I prefer either a parking light bulb (filament type not LED), or a diode in the excite wire because it is convenient. The most likely problem is the internal diodes in the alternator. If one of these has gone bad, it will pass current in the wrong direction letting the battery go down.
The remaining diodes will work well enough to charge the battery when used.
Use a multi meter with a built in amp meter as follows:
Disconnect the alternator big wire. Set the amp meter on its highest amp setting (you may need to move a lead to a different spot on the meter) connect the amp meter leads with the black on the post of alternator and the red on the wire you disconnected. The reading should be less than 150 milliamps. You may need to adjust the meter settings to get an "in range reading". 1000ma equals one amp, so 150 milliamps equals .15 amps.
If it is more like one amp, the the battery will drain in approximately 50 hrs (this depends on the battery and is not a hard number)
If it is less than the 150ma, the diodes are OK.
A battery with a cell going bad can also self discharge. This mimics the situation you have. Even new batteries can be bad. JimN