so my wonderfully reliable, mkIV engined herald, has an little quirk. If I work on it on tickover ( eg, when its Moted ) The no 3 plug (counting from front) fouls up. I then have to remove it and clean all the thick black crud ,off. All others are spot on. Its running an standard head ( and has been for twenty two yrs, no additives used ) so guess that may be the problem. I do around 1500 miles a yr. Thoughts ? Not lifted the head so havent checked rings.