A couple of months ago my mini was running on 3 cylinders,nbr 3 plug was fouled and shorting out.
Today the same thing happened,but now its nbr 3 plug.
Changing the plugs fixes it every time.
I am using NGK 7ES plugs.
What is the cause of the plugs shorting out.

_________________
1967 Morris mini Deluxe
1968 Buick Riviera
1979 HZ Kingswood
1979 CM Valiant
1982 Jaguar XJ6 series 3
2002 VX HSV
2013 Mazda 6
2018 Gt Mustang

When soot builds up on the plug, the spark will short out under compression rather than jump the spark plug gap.

Try a hotter plug. NGK spark plugs get hotter as the number gets smaller. A stock heat range for minis is the BP6ES for a start, but you can also try a BP5ES as well. The hotter plugs help to burn off any sooty buildup before plug fouling occurs. Going too hot however can have consequences in that the engine may be more prone to run-on, so go only a step at a time.

Tuning may also be an issue, a richer than required mixture will cause the same problem. The middle two sparkplugs can often foul up on a lumpy cam at idle.

_________________
SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.