Assuming this is an aircooled:-
Have you tried the plastic plate 'insulation plate' under the carb? If this has a crack or leaks it can cause the problem?
Have you tried putting the old plugs back in (OK I know you re not an idiot, but that is what caused the same problem with mine, a faulty new plug).
Condenser, again?
Have you got a really strong clean spark?
Carb, jet blocked?
If not I would go for the fuel pump. That is the normal problem