If you can keep it running by pumping the primer ball or if it will
run if you keep a lot of choke on it you might have a partial blockage
in the carb. You could try draining the bowl and spraying some
varnish remover into the inlet, but it is better to strip the thing
down and fire cleaner through all the little passages. That isn't
hard. While you have it off take the fuel pump off and check that it
hasn't gotten clogged, that the diaphragm has no holes and that the
spring is still good.

-- Tom.