Now stop and think. Gas is a fluid and where can it flow into the block from? Now you have a new carburetor so what other location could it be coming from? On anything I own or worked on the gas first goes to the fuel pump, so that other place is there other then the new carburetor you installed? I take it that is happening when it sits over night. If that is the case the fluid (gas) has to syphon from the tank into the block. My own thought on it would be you still have a carburetor problem or fuel pump??? what other connection would you have to the block?