close the little hole in the choke plate with your finger whilst choking and flipping. Once fuel is into the carb, all will be well. If the problem persists, check out all tubing. The fuel should stay there and not run back from the carb.
If it does, check your carb very well. See http://www.prme.nl/forum/viewtopic.php?t=10 for understanding the Walbro.