As long as the engine is warm, the choke should not be engaged, but it would be an easy thing to check. When it is colder and the engine is running good, pull the aircleaner and observe the choke position. It should be wide open as long as the engine is warm. Check to see what position it is in during hot times of the day too because your choke may not be fully disengaged.
My money is on the cooler denser air. I know mine runs best on cool days with moderate humidity.
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