Fairbanks Morse 'Z' 3hp throttle governor ignition timing

The manual I have for this engine does not give any info on ignition timing, just exhaust valve timing.

The engine has the correct Fairbanks impulse magneto for the year of the engine.

On other engines I own that have impulse style magnetos installed the mag filcks producing a spark at TDC then advances as RPM rise, on this engine the mag flicks a little way after TDC, can anyone tell me the correct ignition timing for this engine? thanks.

Martyn