Is your plug cap for a CM6 plug? If it is for a taller plug, it might bottom on the head and leave a gap at the topfor RFI to form. If the cap goes over the plug hex, it will normally cause a small gap. I had to wrap an ignition in metal one time. Get a magnetic tin can(not aluminum) and wrap it around the ignition. I used tin snips to fit it. Then solder a wire to the shield and ground it to the motor.