Not sure if it's the left one but it probably is on most single switched setups as it's the furthest position to the right, which would suggest the right mag is earthed.
Only one mag works at start. The impulse mag works by delaying the spark (retarding the ignition point) and then "flinging " the magneto innards round more quickly than the engine speed would suggest. It then plays catchup so the a fatter spark occurs at start cranking speed.
If the other mag worked too and it managed to make a spark at the normal point, which would be too advanced for the low engine speed, it might cause a kickback, which can damage people if it was a "hand swing" start and possibly the crankshaft, due to reversal of loading.
Last edited by ShyTorque; 14th Nov 2004 at 20:14.