Disconnect the fuel line from the carb. Remove the vacuum pulse line, and using a temporary tube, 'pulse' it using your mouth (suck, don't blow!).
If the pump operates, you have a blockage 'downstream' of the pump - probably a stuck carb float needle.
If the pump does NOT operate, you have a blockage/restriction 'upstream' - in spite of you blowing back up the pipe, it's possible for the fuel line to disintegrate internally and look fine on the outside.
If you get a mouthful of gas or oil, you have a ruptured pump diaphragm.
phil_saunders(a)bigfoot.com
Phil Saunders, June 2009
