Fordson Magneto Question

[namdc3]

I have a 6V battery backup on an F to aid in easy starting. It will start on mag when warm, but it starts easier on battery. There is a knife switch from a hot shot battery box that allows me to switch from the magneto to battery or open the knife for off. The original setup just had a single wire from the magneto post to the coil box, and the later tractors like mine have a second wire from the magneto post to a ground post on the extreme end of adjustment of the timing lever. This allows the operator to ground the magneto and shut the engine off from the operator's seat. See this post for someone else's photos. http://www.mtfca.com/discus/messages/70 ... 1505813306

One day I forgot to switch the knife switch from battery to magneto. I later grounded the magneto through the timing lever post while the coils were running on battery. The engine sputtered and fluttered around without dying. I un-grounded the magneto and the engine returned to running like normal, on battery. Worried that I had just scrambled the magneto magnets, I switched the knife switch to magneto, and the engine continued to run like normal.

What happened when I grounded the magneto while running the engine on battery? Why did the engine run roughly? The magneto was still disconnected from the coil box input, I was just creating two circuits with a common ground. Did I have a near miss of scrambling the magnets? I think I'll mistake proof this system of mine, but I'd like to understand the risks as is and what happened electrically. Thanks.

[Henry K. Lee]

Sounds like you were cancelling out half of your sine wave from the magneto with the pulses from your battery. Since it was in motion no real harm has happened. It is still starting OK, correct?

Hope this clarifies a little,

Hank

[namdc3]

I was just thinking perhaps the magneto was back feeding into the negative side of the battery, with half the sine wave cancelling the positive feed of the battery to the coils. It might be worth mentioning that the magneto's ground point and battery's ground point were pretty close together. Also worth mentioning is that the engine running rough wasn't a function of poor commutator timing because moving the spark lever from the button to just off the button doesn't change the timing due to flex in the linkages.

I actually don't think I've tried to start it on mag since that happened. I do know it still runs on mag. It would be interesting to do a voltage test on the mag and see what the mag's output is, although I don't have a data point from before.

Nik