The timing light is a 6/12 volt combo. The manual says use a 12 volt power source and add a ground from the 12 volt vehicle to the 6 volt vehicle.
Thanks for the input, it is a 6 volt coil.
Stuart the light works fine on my 12 volt trucks, no problem there. Ill try a non steel cored wire and see if that effects anything.
I really appreciate all the input rather then the whole 'guess the timing' routine. Thanks, Jeff