Hi Seanw, your symptoms are consistent with a blocked jet. Spray with carby cleaner and poke through the main hole with a piece of bread tie wire or a strand from a wire brush, spray again and blow with compressed air. It should see it right. While you're there remove the primer face and clean float chamber and primer face too. Be prepared to replace the main o-ring as after its disturbed from sitting there for such a long time it will most ptobably leak on re-assembly.
Let us know how it works out for you.
