Many understand that the Autolite AR3910X is actually to cold for most stock class applications.
Years ago when I first heard about the Autolite X PLUGS I immediately consulted a heat range chart. The plug is actually available in three different heat ranges. In addition to the 3910 there is an AR3922X, and a AR3933X there was a very limited production AR3911x but I don't think you will ever see one. I have always thought it was laughable that so many promoted the 3910X and apparently never bothered to consult a heat range chart, even Briggs and Stratton failed to do so when writing the rules for the LO206. Tired of fouling out this plug? Try the hotter ones. They are readily available. I have been using all three for a very long time.