The obvious first thing is, have you tried different plugs? Also when earthing the plug, the end of the HT lead must be clear of the earth point. You should get a spark from just the end of the HT lead when held 1/8" or so from an earth point. Ron
PS I'm not sure how you test the HT with a multimeter? The spark generated by the magneto is in the Millivolt rage...Hence the shock if you touch it. Maybe someone with more electronics experience can advice?