For some time my 1970 BMW 2002 had an annoying roughness at idle. The carb was re-built, dwell and timing variation checked, and compression tested, but to no avail. However, it was noted that No. 3 spark plug was always carboned and was difficult to remove, even though it had been torqued to spec value.
By way of some now forgotten logic it was deduced that the spark plug was not fully inserted. It was found that the plug would register 20 ft-lbs of torque 5-mm short of being seated in the head. Apparently, this was caused by the metal sealing gasket being cocked on the threads during previous assembly, thus preventing complete insertion, and allowing a carbon build-up in the last four threads of the plug bore. The carbon would refuse the plug at that point at each successive change.
The solution was to carefully run a 14 x 1.25-mm tap through the plug bore (by hand!) to clean out the carbon deposits. The plug seating surface on the head was also checked for any retained metal sealing gaskets. Then, after applying a thin coat of anti-seize compound on the plug threads, the plug was placed in a plug socket with a short extension and threaded into the head by hand for eleven revolutions. (If you count less than eleven turns, the sealing gasket may be cocked; if more than eleven, you miscounted.) Then, it was torqued to spec.
As a reminder, never use motor oil as a spark plug lubricant; it will 'caramelize' at combustion temperatures.
Author: John Greenwood