The invention relates to a portable, hand-operated working machine (1) with a tool which is rotatable by an engine through a driving shaft (20), and a transmission comprising a centrifugal coupling, a driving belt (8) and a driving wheel with a groove (7) for the driving belt fitting in said groove, which driving wheel comprises an outer (3) and an inner (4, 4′) half of the driving wheel, wherein the outer half (3) of the driving wheel consists of an inner wall of a coupling drum (11) being part of the centrifugal coupling, which drum is rotated by the driving shaft when the centrifugal coupling is in the operative state. The characteristic feature of the invention is that a fan (14) is provided inside the driving belt in the axial direction, coaxially to the driving shaft, and arranged to rotate at least when the centrifugal coupling is in the operative state, which fan has fan blades (15) extending at least partly radially beyond said groove for the driving belt, which fan blades are designed to suck cooling air in the axial direction towards the space outside said groove for the driving belt and towards the drum of the centrifugal coupling.
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