I have a mini 1100 with 1.5 inch carb and basic extractors.
The number 3 plug fouls up regularly.
Replace the plug and it runs well for several days – Car only does short trips.
Compressions are all even.
Valve clearances and lift are also good.
I have also changed the coil, distributor and leads making no difference.
What can be causing number three to foul up given the compressions are all good?

Is it oil or carbon fouling?

Could be worn valve guides or ring problem.

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Plug looks wet like oil or fuel.

If they look wet (and a bit crunchy) then it is Oil (although have your check / replaced yr leads?).

Your've done a compression test yeah, but have you done this at operating temp and a dry and then wet test? (dry is just that and for a wet test squirt a little bit of oil in to each cylinder then test again).

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My 998 mini has had a similar problem but fouls number two plug. It has the same carby and exhaust set up as yours and also only does short trips. I have found that if I keep the fuel fresh and have at least a quarter of a tank of fuel I don't have the fouling problem. Having said that it will probably stuff up tomorrow.

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