Bubbles

The pressure in the Airbox and the pressure in the Float Bowl should be exactly the same.

Whenever the throttle is changed the pressure changes and the vent line allows the pressure to equalize while the RPM varies.

This is the purpose of the Vent Line.

Bubbles moving from the Bowl to the box indicates that there is a pressure differential that is trying to equalize but equalization is never happening.

The air in the Carb throat, and therefore the Airbox is always less than the ambient pressure surrounding the engine.

If the Float bowl has an air leak to the outside ambient pressure; this will raise the pressure in the float bowl, causing a rich mixture and rough running while at the same time the added pressure causes a continuous flow from the bowl to the airbox as it tries to equalize.

The normal vibrations and sloshing of the fuel in the bowl cause small amounts of fuel to "Bubble up the vent line; again adding to a richer mixture.

I would investigate the possibility of an air leak in the float chamber assembly.

Always 91UL, for 600 hrs now.

Replaced leaking cork float bowl gasket with a rubber one and no more bubbles. Still intermittent shudder at 5000RPM. Think carbs are wanting a rebuild.