Jeff,
See if your timing light is marked for 6 and 12 volts. In the past, timing lights were all made to work on either voltage, and were marked as such. I haven't looked at newer lights, but it may be that 6v timing lights are not in the current crop of tools available from auto supplies and tool stores. To get around your problem, and avoid getting shocks, hook your timing light to a 12 volt battery while you time the 6v engine.
As for your metallic spark plug wires, they were original equipment for a long time, and using them on your vehicle is all right. They do provide a little more spark current than resistive wires, hotter spark so to speak, and don't drop secondary voltage as resistive wires do. Their primary drawback, as you noted, is radiated radio-frequency noise that causes crackling in AM radios and "snowy" bands in television sets. Those concerns were the origin of TVRS (television and radio supression) ignition wires that later became just "RS" or "supression" or "resistor" wires. The 7mm diameter is the proper size for your setup, although a larger diameter won't/shouldn't cause any trouble.
Your ignition coil is in fact a transformer (technically an autotransformer), but that point is immaterial for your timing light issue- unless you have changed the coil to something other than an original-equipment replacement. If you changed the coil, it is a remote possibility that you installed a 12 volt coil with an internal ballast resistor. That combination with your 6v ignition system will cause you trouble in the form of a weak spark, and cause your timing light to work as you describe.
Harvester
