It depends how the Extreme is wired up. Normally with a magneto (I'm assuming it still uses mag, not an alternator because of the push start heritage) you stop the engine by shorting the coil to earth, rather than interrupting the circuit to the coil.
This can mean that the engine may stop if the wire to the kill switch is worn and touches earth (the chassis)
Or that the switch itself gets contaminated or worn and shorts to earth, (I've had a switch where the grinding of the switch coupled with WD40 made a conductive paste that shorted the switch)
The reason you don't interrupt the current to the coil is that if it bridges the gap, you cannot turn the engine off except by pulling the plug cap off (painful) or choking teh engine to death.
However, an old failure of DD aircooled was to have the wire to the coil LT break inside its insulation, giving a spark that worked under ideal conditions.
The reason it fails when trying to pull away, but runs at idle or high speed is that when trying to pull away, you change the mixture from the ideal at idle which can survive with a weak spark to a much richer mixture which requires a good spark to keep it going.
Once the engine is going, it is usually warm (which helps vaporisation of teh fuel, and if some of the sparks fail, the engine momentum keeps it going till one 'catches'.
Another variation of the theme is the faulty spark plug cap, but if the engine has been recently refurbished that should have been tested.
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