I've also had a couple of Bynorm/GA primer caps that seem to be either warped or that the O ring land is too small. They leak even with a new O ring. Not to mention the needles!!

It probably isn't justifiable for a lawn mower, but a Mikuni or Tillotson carby conversion should make a difference.

I have heard that the older Victa G3 carby is well suited for kart applications etc.

It would be interesting to find out what sort of carby Ron Wheeler used on the Skycraft Scout that used modified Victa 160cc engines. https://en.wikipedia.org/wiki/Skycraft_Scout