Customer with late coil ignition model Matchless G15CS – very similar to N15CS, Atlas & P11 – even early Commando – wanted to know if I could give him a measurement so he could stick a rod into his plug hole to set his timing. Here’s my reply…
That would not be accurate enough because a piston doesn’t go down the bore at distance exactly in synch with the rotation of the crank and also the plug threads are not exactly 90 degrees to the piston top. Also piston crowns vary a bit in height which would mean a differing starting point.
Add to this, the optimum timing varies according to the gas you regularly use. These bikes were originally designed to run on 98 octane fuel – much higher than that presently available. I would suggest you set the timing according to the specifications and then
take it for a ride. If on roll on acceleration from say 40-50 mph you hear and sound of detonation (pinging) then rotate your pick up plate say 1/16″ in the direction of rotation of the magnetic rotor and try again. This will effectively retard the ignition slightly. Keep doing this until
you get rid of the detonation.
We do carry and contact breaker mounted timing disc (98-0818CEI $27.55 Canadian) which you would find useful for this purpose.