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What are the colored dots on the SUs?

Billcapp, good question. To finetune the mixture there is a screw underneath the carb with 8 or nine sectors. I gave a different colour to each sector and painted the same order of colours on the top. There I marked the colour that just is tuned and drive for some miles. Than I looked at the colour of the spark plug and if necessary I tune the mixtzure leaner or more fat and mark the new colour on the top. In the end I have marked the best mixture for each carb and noticed it in my manual.