My mopeds have always run best in the late summer, early fall. I think the cool dry air is best.
The humid hot air or cool dry air can greatly affect your moped performance--- the thick humid air must be almost like a liquid when your card tries to suck it in....
(I'm guessing here--not an expert or a meteorologist....)
I think of it this way-- You've got a tiny engine, going maximum throttle-- working it to the max every day.
Wind, weight, poor quality fuel, temperature, humidity, condensation in tank (Due to cool nights and HOT days) all of these can have an impact.
And you notice the difference, cause one day you're going 27 even, and the next day, you hit 33.... the only change is the weather.
I've also heard you need to adjust your carb, to help 'adjust' to the seasons changing. I don't know how good that info is-- a Harley guy told me that he does it on his bike when the seasons change.
I have no doubt that even Low pressure or high pressure systems have the potential to affect performance.
I work at a huge company--- the building is almost totally sealed, except for the ventilation systems.
How's the weather? In Maine, it's been cold, but this week is getting sunnier and a lot warmer.
When there is a huge daytime atmospheric pressure drop or increase, and you leave the building you open the door, and the air either rushes out of the building, like air from a baloon, or rushes into the building like someone taking a deep breath....
Would high pressure force more air into your intake? Would low pressure make it harder to get enough air?
On a huge engine-- little changes don't seem to have an effect--- but a moped is so small, and always running at "top end" I think subtle changes can have a big effect-- and you notice the changes more, because you're always at full throttle.