I would suggest that any engine that fouls the plugs under the stated conditions has the idle mixture set too rich.
An easy way to check is to note the momentary rise in RPM as the mixture is pulled to idle cut off. There should be a rise of 25 to 50 RPM. If the rise is excessive the idle mixture is too rich and if there is no rise the mixture is too lean.
A friend was recently complaining about his plugs fouling on the taxi. I found a rise of over 100 RPM so we had the mixture adjusted and his plug fouling problem went away.
BTW the proper way to do a "burn out", that is clear plug fouling, is to go to runup RPM and lean until the engine is so lean it is running rough. Hold that for about 15 second then return to full rich and check the mags. If that does not clear the plugs you have other problems.
Avoid excessive running with the mixture grossly leaned. If 15 seconds of lean running won't clear the plugs, more time won't help.
Last edited by Big Pistons Forever; 27th October 2013 at 16:23.