It isn't actually "rich"- - - -but failing to ignite the mixture efficiently causes the engine to put a lot of unburned fuel down the exhaust pipe and sometimes makes the spark plugs get sooty or fuel-fouled. Half-donkey electronic ignition systems don't always fire the plugs as effectively as a plain old mechanical point system.
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!