I had a couple that wouldn't start lately too but I knew the needle wasn't getting stuck which just left the primer as the problem, one primer if I held my finger over the plastic hole on the inside then pushed the black primer bubble I could hear the air escaping from the rubber bubble to the plastic mount so that cap is useless and the other cap the primer tested good (no air leaking) but would still not prime ,the problem was the cap o ring was leaking (if you measure your cap where the o ring sits and it's slightly under size to the old cap the o ring may not seal, sometimes the o ring will leak air at the top so fuel doesn't drip out but the primer doesn't work. I put the same cap on another carby with a new O ring and the primer worked well. These primer caps were second hand .
As we know the fuel bowl is sealed and the air vent is the small hole in the rubber primer bubble ,pushing the bubble pressurises the fuel bowl and fuel is forced into the main jet.
Not sure why so many are leaking for you Norm ,I put a metal needle and clip into a Victa 460 Utility today and it worked well but I did notice some floats are different ,one float looked almost bottomed out with the needle closed so I used a different float and the float looked in a lot better position.
