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Is this by any chance the mower in question? its hard to give good help on a machine if one cant be definate on whats being worked on.

the ignition system is pretty simple, the coil, the ignition module, spark plug, the wiring and the cut off rubbers. The coil can be quite dear to buy new but there are abundances of second hand units out there. At the same time try a second brand new spark plug, I have had them faulty out of the box before. Is the ignition module being used a long rectangular gold one or a small square silver one? The old gold coloured ones are notorious for being faulty, however its worth making sure is earthed well and RIVETED to the motor not bolted or screwed as that can rattle loose and cause issues down the track.