Sime,
Check the carb to head rubber manifold for splits and make sure the carb hasn't popped off it's flange.A bit of "smoke"(it's vapour really)is pretty normal when you open the oil cap when the motor is hot,don't worry.If still no joy,re-check the valve clearances to be sure one hasn't tightened up.
Jon.
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Just going for a short ride on my bike....
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