Since a throttle hole for narrowing a scavenging passage is provided on a partition for closing the scavenging passage, the throttle hole of a desired size can be drilled at a desired location on the partition after preparing the scavenging cover having no throttle hole on the partition. Accordingly, the size and location of the throttle hole can be easily changed in conducting a test for finding the most appropriate size and location of the throttle hole, so that the most appropriate size and location of the throttle hole can be easily found.