Took all the lights off- need a good way to cut off the power to the coil-  The switches I have are both momentary two wire switches that mount on the handlebars.  I think they are meant to ground out the pulses to the primary side of the high tension coil.  What has worked for you?  Details please!

 

We have to use lanyard killswitches for trials competition here and popular lanyard kill switches are made by Leonelli and Jitsie and have a magnetic action

https://www.google.com/search?q=leonelli+magnetic+lanyard+kill+switch&rlz=1C1GCEB_enAU911AU911&oq=leonelli+kill&aqs=chrome.3.0j69i57j0i22i30l3.11568j0j7&sourceid=chrome&ie=UTF-8&safe=active&ssui=on

All my bikes require the killswitch to earth out the LT circuit of the HT coil to stop the motor. These kill switches I mentioned have two wires and either of them can be used to connect to an earth point. It's best to run the earth wire from the kill switch to an earth point on the main frame of the bike, to avoid sending current through the steering bearings, which could otherwise damage the bearings. Most people connect the kill switch earth wire to where the HT coil connects to the frame. Most people connect the other wire from the killswitch to the LT wire near where it connects to the HT coil.

2 hours ago, feetupfun said:

We have to use lanyard killswitches for trials competition here and popular lanyard kill switches are made by Leonelli and Jitsie and have a magnetic action

https://www.google.com/search?q=leonelli+magnetic+lanyard+kill+switch&rlz=1C1GCEB_enAU911AU911&oq=leonelli+kill&aqs=chrome.3.0j69i57j0i22i30l3.11568j0j7&sourceid=chrome&ie=UTF-8&safe=active&ssui=on

All my bikes require the killswitch to earth out the LT circuit of the HT coil to stop the motor. These kill switches I mentioned have two wires and either of them can be used to connect to an earth point. It's best to run the earth wire from the kill switch to an earth point on the main frame of the bike, to avoid sending current through the steering bearings, which could otherwise damage the bearings. Most people connect the kill switch earth wire to where the HT coil connects to the frame. Most people connect the other wire from the killswitch to the LT wire near where it connects to the HT coil.

I have the switch between earth ground and the low side of the HT coil where it joins the wires up from the magneto.  The problem is that no matter whether the switch is on or off- there is still spark at the plug.   I'm thinking that maybe the voltage is arcing across the switch contacts inside the switch when it's open.  Does that make any sense?  How much voltage would you expect to see on the low side of the coil when the magneto fires?

No it won't arc across between kill switch contacts. It is low voltage and a proper kill switch is designed to do it's job. I've never measured any but I would expect it would be less than 30 volts.

First off you need to test if the kill switch is actually working or not. I suggest you measure the resistance through it with both wires disconnected from everything else.