Cat D4xx, Pony Motor Magneto Hack

When I first got my 1953 D47U I spent a lot of time trying to get the pony motor to start and run reliably. Because I was getting spark at the plugs, I expended most of my efforts on carb cleaning and adjustments but made little progress. I could generally get the thing running on a warm day, on a cooler day with a hit of starting spray, but never on a cold day. Then I remembered what a sage mechanic told me years ago. “99% of carb problems are in the ignition system”. I realized that although I was getting spark, I was not getting a great hot spark.

I cleaned up the points and replaced the plugs and condenser, but still was getting a weak spark. This was many years ago, so I don’t remember the exact metrics, but an ohmmeter reading on the secondary coil of the magneto revealed low resistance to ground.
On one of the old equipment sites (possibly this one) I saw a posting from New Zealand about using an auto coil instead of the magneto, but with no further explanation. After spending some time thinking about it I came up with the following system which uses two auto coils, one for each cylinder.
The system connects each spark plug to its own coil. The hot sides of the coil are tied together and wired to the positive battery terminal (I have a starter motor on the pony, if you only have pull start you will have to add a battery). The negative terminals of the coils are tied together and wired into the points using the wiring that went to the original pony kill switch, which shut off the pony by grounding the hot side of the points. The kill switch still functions as a kill switch by opening the connection from the positive side of the coil to the battery. I did this several years ago and don’t remember explicitly, but I imagine I removed the wire that connects the points to the primary side of the magneto, if I didn’t I should have. The results of this hack is a strong running/starting pony motor.

The old magneto and rotor are still in place, but only the points and condenser are being used. The only anomaly is that there are two sparks for each four cycles instead of one, the desired spark for the power stroke and an extraneous spark during the top of the exhaust stroke.

Good information!
What is your battery voltage, still 6 volt or 12? And which coils did you use?

[quote="ccjersey"]Good information!
What is your battery voltage, still 6 volt or 12? And which coils did you use?[/quote]

The starter is 6 volts but the battery is 12 volts. I used two generic 12 volt coils from the local auto parts store. While the 6 volt starter easier survives the 12 volts I would not recommend using 6 volt coils with a 12 volt battery, it would fry the primary windings sooner or later, probably sooner.

I have wondered why a distributor system wasn't offered for a electric start pony, sure it allows rope starting with a dead battery, but it still would be useful in some cases. These days a useful mod would be a 12v dewalt battery (or two) plugged in would start the pony & fire a coil. That's what I would like for my lawn mower.