I also noticed it at hotter days.
Mostly happened when you where about to move after waiting at a traffic light.
Dealer told me it was normal.

It starts right away, so no problem.

@KEV: you cant adjust idle rpm as its all controlled by the computer (that was what the dealer told me)

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Derbi Senda DRD Sm "04 (black) => BMW F650GS "04 (yellow) => Yamaha xt660z T�n�r� "09 (blue)