Letting the spark jump from the wire to the plug allows voltage to build up and jump instead of just leaking away as it develops. That's usually a sign of a carbon-fouled or oil-fouled plug. Also look into very wrong fuel mixture, either too rich or too lean.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!