• If you log in, the ads disappear in the forum and gallery. If you need help logging in, send request for help to: webmaster@southernairboat.com

Magneto wiring


I had a GP250 and blew up the motor. I was able to find a replacement. I switched out motors but was getting no spark. So I switched out the coil, condenser and points from the motor that I knew ran (until it slung a rod) into the replacement motor. Unfortunately, I did not take enough notice where all the wires went. I have watched you tube videos and believe I have it right. One leg of the coil goes to ground, the other goes to the points, and the condenser wire also to the points. The wire I am unsure about is the one that grounds out the magneto to kill the engine. Does it connect to the points too? Who knows if this is going to make it run even if I get the wires right. Will probably need professional to set timing etc. Any help you can provide will be more than appreciated.

Slidin Gator

Well-known member
This is the "P" terminal, "P" for points I assume, but it's where one leg of the coil primary, the condenser (capacitor) and ground out lead tie together. When the points are closed, the coil is grounded. When the points open, that's when it builds current in the coil and creates a spark.

When the "P" terminal is switched to ground it is the same as if the points never open. The I suggest 14 gauge minimum, prefer 12 for the ground out wire, it should be at least as large as the coil wires. Also note that this wire is like a giant antenna generating radio noise when the mag is running. It sees voltage spikes upwards of 150 volts, the wire and grounding switch should be rated for 250 VAC.

Shielded aviation setups use coax cable with the shield grounded by the rusty nut (ferrule) and the center wire to the ground switch to eliminate this source of radio noise.

Last edited:

Gary S

Well-known member
Number 12 wire? Why so big there is little current and it does not take much to kill a mag. Most I use is 16 and have used 18 in the past. Airplanes do not have 12 going to the mags.

Slidin Gator

Well-known member
My logic here is primarily related to corrosion at the connector, bigger wire has more contact area in the connector crimp. Also more wires to break before it does not work.

You are correct that smaller does work, I said at least as large as the coil wire. I prefer larger for reliability.