Check the battery is fully charged and holds the charge-ie does not go flat in 24 hours.Check the earth wire that is attached to the frame -often the steel washer gets rusty.Sand it bright and shiny and reattach.Take the tank and seat off and check that none of tye wire are pinched or bare.If the seat is the flip type wires can get trapped in the seat and short out.Tape any bare sections.Take out the plug and ground it to the head with a metal clip.Wait till dark and then crank the kick start-the spark should be nice and blue and about 2mm wide.If no or weak spark(yellow or very thin)try replacing the HT lead cap or the whole HT lead(the fat fire that goes to the plug).If still no joy take it to an auto electrician