Ken,
3 different engines with the same problem sounds like an issue with fuel supply -

1. Check the fuel lines for holes or cracks including the fuel line thats connected to the fuel pickup (clunk) thats inside the tank.

2. Check that the clunk fuel line IS connected to the carby/needle valve and moves freely inside the tank.

Sounds simple but I've seen these things overlooked.

Hope this helps.
Ossie