Yep, the metal cable for the throttle linkage has to go first. Then, if the spark plug wire is not shielded with a metal braid, you need to add one. I have had to do this before on magneto ignitions. You can carefully strip the insulation off of regular coaxial tv cable, then remove the copper braid and use that as a shield. Just ground it on ONE end; not both. For example ground it to the engine at either the spark plug end or the coil end, but not both. Finally, kill switch wires can also transmit rf interference. Keep yours as far from the radio and/or antenna as possible, and keep them short.

This all assumes that you are using a resistor spark plug. The plug you are using must have an "R" in the number, indicating that it is a resistor plug.

Do not fly it until you fix it. It will not "get better" in the air and you could well lose the airplane.

AV8TOR