I have bubbles in one rancor also, they disappear when I switch to the other tank. This means a leak in the supply lines. I haven’t found the culprit yet, but I suspect a hole in the standpipe in the tank.
From an old MB&S magazine look a this Tip: " If bubbles rise up through the bowl, the pet cock is leaking. Should boubles circulate from the intake down across the turbine area, there is a leak between the fuel tank and the filter ". I hope this helps.