This is a 'classic' engine. It's possible that the diaphragm between the carb and tank has ruptured - but it's more likely that the check valve inside the carburettor has stuck. This is a non-replaceable part, but sometimes a sharp tap on the carb will free it off.
If not (and the tank is full), the only cure is a new carb. Carb cleaner is not an option - it will dissolve the plastic body before it dissolves the crud causing the problem.
phil_saunders(a)bigfoot.com
Phil Saunders, September 2009