Fuel supply assembly for a two-stroke crankcase scavenged combustion engine that is operated with layered scavenging, said assembly including a housing (2) and two parallel channels (3, 4) extending through the housing, viz. a first channel (3) for a fuel/air mixture and a second channel (4) for auxiliary air, a partition wall (5) extending in a plane between said channels and arranged both upstream and downstream of a butterfly valve, and the at least one butterfly valve (7,8) with a butterfly (disc) (14) pivoted in the housing, said butterfly having an axis of rotation coinciding with the plane of the partition wall, a downstream side, an upstream side, and a protruding section (41) which extends across the butterfly along the axis of rotation at least on the downstream side of the butterfly. Specifically an edge of the partition wall, or a sealing member on the partition wall, contacts or nearly contacts said protruding section at least on the downstream side of the butterfly, securing a seal between the two channels in all positions of the butterfly sufficiently efficient for preventing any non-negligible passage of fuel from the first channel to the second channel on the downstream side of the butterfly.