With a Weber, pumping the throttle squirts gas into the manifold making it over rich. When hot it should start without touching the pedal. Don't pump it and hope, you'll need to get rid of the excess gas before it will fire up. Pedal to the floor and wind it on the starter until it fires, then slowly come up off the pedal. SU's don't have an accelerator pump so don't suffer from this. Not to say they can't suffer from similar problems though.

If you want to try a non return valve they are commonly used on boats with inboard engines so try your local marine supplier.