The most common thing that causes engine run on is idel speed too fast. Timing retarding the timing slightly will tend to slow the idle speed down and is the reason that setting teh timing back at times works to control run on. As for timing itself causing it it can't. Went the key is turned off the spark to the ignition is gone so timing from that point is a none function.
Detonation or hot spots is another cause to look into.