Let me guess is it the lower cylinder that has gas in it? If so the float needle or seat are worn. Or there is some debris in between the needle and seat. Chances are the fuel tap doesn't turn off all the way. So by gravity the fuel runs to the lowest point. My MC-18 would fill the lower cylinder and pipe with fuel. I changed to a set of MC-21 carbs and new fuel tap and the problem went away. You may just have something in between the needle and seat. Fuel filter would be a good idea.
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