[Kenth]

There is NO vacuum in aircleaner or crankcase since they are vented to atmosphere at all times without restrictions. With engine running vacuum is present in intake only (unless throttles are wide open on heavy load).

What you see is atmospheric pressure rushing into the void under the partially closed throttle plates. This atmospheric pressure pushes fuel through the carburetor circuits and crankcase fumes from the CC vent tube for combustion.

HTH