i did this once and ended up with a 10km ride with most of it uphill...with half a cup of fuel for assistance...
Try turning it off when you're
almost home, as Tedd meant, instead of 10km from home.
What did you expect???
Personally, I agree with Tedd - NEVER leave the fuel petcock 'on' while the bike is parked.
As I mentioned earlier, if the needle and seat leaks the crankcase can fill with fuel, not desirable.
This is separate to the actual problem.
All the talk about float level and so on means nothing in this situation. There's a reason there's a fibre washer originally fitted to the carb's fuel inlet tube. Regardless of float level, if there isn't a fibre washer fitted, fuel
will leak. The fuel doesn't sit obediently at the float level all of the time. It is splashed and shaken around as you ride, and will cover the base of the inlet tube during (RH) cornering. I've had that leak in the past, on a few carbs, on a few different types of bike, (not MBs). The float level was set correctly in all cases.