Lots of solid state ignition manufactures are using the plug wire shield, and shield cap as the only ground return for the plugs spark voltage. A bad idea IMO. With that setup, the shield to cap, and cap to engine connection becomes a very vital connection. Any looseness in the connections, and you have lots of high voltage static outside of the shielded system. All of the magneto systems have the High voltage ground return connected directly to the engine frame. I'd highly advise running a separate heavy ground wire from the ignition modules ground to the engines frame. That takes the load off the shield wire, and lets it do its intended job. Without the separate ground, the shield can break, and the engine will continue to run. The high voltage just jumps the ground gap, just like it does at the plugs gap. The only problem is, you may not no it until its to late.

Kevin