Originally Posted by Darth Vader

Check if you have a torn manifold gasket.
If still cannot trace the leak, cleanup the engine, then run it on the starter box to see if you can detect where the leak is. Doubt it's from the carb coz carb would be having negative pressure when running. Air will leak into the carb but fuel will not come out.
Probable areas are front bearing, back plate, manifold.
Maybe also the engine head if it's not screwed tight to the crankcase, but engine won;t be running properly if that's the case...