As the humidity (water vapor in the air) increases, the percentage of oxygen in a volume of air decreases. This causes your engine to run slightly rich. On a foggy day, it can get really bad. You probably need to touch up the low end by leaning it out slightly, 1/8 turn in or less on the mixture screw. The problem with adjusting for an occasional high humidity day is you have to adjust back when the air gets less humid. If I have a really good running engine, on one of those low clouds, "the fog has just lifted", really humid days, I tweak the high end and accept the low end so I don't have to reset the next weekend.

If the engine is new, it also will get better with more running. I have an Evo .46 and after a gallon of fuel, they are easier to keep adjusted. They are partially broken in at the factory and run well out of the box, but they aren't completely freed up. As you get more running time, you'll have to adjust less.

Some engines will never lose the slight hesitation. I haven't noticed this on my Evo.