We had exactly the same issue at the last race of last season.
Richard from UFO put us onto the spark plug gap trick towards the end of practice and it worked ok.
For the winter cup we came a little better prepared and had a seperate set of wet weather spark plugs already with a reduced gap and we doused the inside of the spark plug cap with wd40 before hand. As a result we had absolutely no issues with the engines at all this time round.
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