I concur with the information from Sla@ts above and here is a bit more.
I forgot to mention that the spark you see in the open air may not be
the spark you get inside the cylinder when the plug in in a compressed
air situation. Engine compression can "snuff out" a spark if the
magneto is weak. Have a "nutter" hold the plug lead 1/2 inch away from
the plug when you strongly pull it over--if it is a blue spark your
magneto is probably OK. If it doesn't fire or is a weak yellow your
magneto could be a problem. Good Luck