First session today in cold, damp conditions. (Minimax) After several (3 or 4?) laps son comes in. Temp only up to 35 degrees and misfire midrange upwards. Taped rad(now more than partial), back out, temp up to 55 degrees but still missing. Throughout morning checked jets, changed fuel pump, plugs etc. but no improvement. Gives symptoms of coil breaking down and slight smoke from exhaust where previously did not exist. Back at home have removed last new plug: perfectly clean, no way of getting any sort of reading! Further investigation through plug aperture shows unusual oily deposit, approx. 1 inch dia. at centre of piston crown, where crown has previously always been spotless. Could "hard" driving at low temperature have broken ring? Any ideas please...........
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