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With the float bowl drain screw tight gas will still leak out of it, could there be something wrong with the screw fit?[/b]
I doubt it unless there is something caught in the seat of the drain screw preventing it from sealing. Possibility if it was opened once and then closed down on a grain or sand or something. Pull out the drain screw out, inspect the screw tip and the seat visually to insure it looks normal. That nipple on the bottom of the bowl is both the bowl drain and the float bowl overflow. If the drain screw is not AFU'd, then the float needle is leaking and not shutting off fuel when the float bowl is full, float stuck, or float level is too high. the causes of you problem are quite limited here.
When cleaning, are you removing the float and float needle and blowing air through the gasoline inlet of the carburetor? Have you closely inspected the tip of the float needle to insure it looks good, rubber tip on it intact? Does it have a float needle even? Looking into the port of where the float needle is installed, does the seat look good? Was the float level 17MM? Could the floats be rubbing against the center tower where the overflow tube is and hanging up? when reinstalling are you putting it back together correctly?
If all is installed well, you should be able to blow with your mouth into the gasoline intake and have air flow with the carburetor upright, if you then invert the carburetor, Gravity should then close the float needle and you will not be able to blow through the gas inlet.