Never rely totally on your timing light as your sole source for timing your motor. Timing lights are typically D.C. and magnetos generate A.C. They also emit E.M.I. (Electro Magnetic Interference). In general, timing lights with mags are one (1) degree late for every 1000 RPM of engine speed. For best results, use a timing buzzer.
Posted off of Don Zig magneto's web page