After checking feed pipes (unlikely if breaking the diaphragm seal sorts it), replace the following in this order (costs pennies) ;-
1) Replace primer. It may be 'soft' due to age.
2) Replace carb diaphragm. (holed, or work-hardened)
If still no joy, there will be a stuck ball valve in the carb body (the only moving part apart from the throttle valve). Some sharp taps with a hard object on the carb body will sometimes make it free up. As a last resort, try carb cleaner (carefull - prolonged exposure will melt the plastic). Failing that, you will need to replace the carb.
phil_saunders@bigfoot.com
Phil Saunders
July 2008