It's just not possible to find a hot enough plug to keep from fouling during extended periods of idling. If you're running at idle speed for more than 10 minutes or so, you're not doing the engine any favors. If you absolutely MUST get to operating temp for some reason other than just admiring the sound of the engine, do it at 1,000-1,500 RPM so you'll be in the midrange or high speed fuel circuit- - - -not idle. Once the engine is tuned, it should be good to go for months, with an occasional 5-minute run just to keep things limbered up. What's the purpose for all the tinkering, anyway?
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!