Small Engines (Lawn Mowers, etc.): Magneto timing, stator plate, inductive timing light

Question
Hi Marc,

I am trying to time a 2 stroke 200cc petrol engine. It has a home built pointerless (no contacts) magneto ignition system. How can I find out exacly when the spark plug will spark as I need to time it to 2.63mm BTDC.
There are no timing marks or rigging holes. Absolutly nothing to sugest we I should set the timing
This would be easy if it had points

Regards

Terry

Answer
That is a tough question because you can certainly measure 2.63mm BTDC with a dial indicator but if there are no marks or indication of stator plate position, you have no way to determine when ignition occurs (like using a "buzz-box" on a points system).  The only way I can think of doing this is creating your own set of timing marks.  Measure the piston position with a dial indicator in the plug hole.  When your indicator reads 2.63mm BTDC, mark the flywheel and a fixed point (both marks as precisely aligned and close together as possible)  Then you can attach an inductive timing light and run the engine with the light pointed at the flywheel mark.  If the flywheel mark is not aligned with the fixed mark (which it likely won't be), make a mental note of how many degrees it is away from your fixed mark.  Then you will have to loosen the stator plate and either advance or retard the position the amount of degrees you noted the timing is out.  Then tighten the stator plate (do not try to leave it loose and bump it as it is likely in close proximity to the rotating flywheel and interference could occur) and check again with the timing light.  This may take a few tries but you should eventually get the marks you made aligned as indicated by the timing light.  This would then have the spark timing exactly at 2.63mm BTDC.  

That's a wordy explanation and I hope you can follow it.  If you don't know how to accurately measure TDC of the piston, there are plenty of websites that can show you or write me back and I will explain.  Good luck and let me know how you make out.