My thinking is it`s probably just the float level is set too high.
If this is the case, even with the petrol tap turned off. The level of fuel in the carburetor will still be too high.
With the bike on the side stand the excess fuel can leak out of the pilot & needle jet orifices into the engine. Also, as the float level setting is out, it won`t allow the floats to actually close the float valve.
This means the fuel left in the fuel line and filter can drain down, refilling the bowl, in turn leaking more fuel into the engine.
You may want to check that the overflow tube isn`t blocked anywhere also. Not just the plastic pipe but the brass tube sticking up inside the float bowl too.
The float bowl vents are a bit higher than the jet drillings into the venturi throat, so any fuel will reach there first.
The workshop section on this site should explain how to check and set the float levels.
Hope you find the problem.
Cheers, Roy.