Is there a vacuum hose connected to the right side carb (rear cylinder)? I think that's the hose in the last pic, and connects to the valve, also in the last pic under the rear of the tank.
If the other hose is connected beside the radiator cap, then it runs to the coolant reservoir.
Make sure the fuel lines don't have any loops or uphill runs to the carbs. They need to run downward or at least level.
If the other hose is connected beside the radiator cap, then it runs to the coolant reservoir.
Make sure the fuel lines don't have any loops or uphill runs to the carbs. They need to run downward or at least level.