I come back to what I said earlier - when they make these planes in the factory, they all start! If they didn't they wouldn't send them out. So what has gone wrong in the years since?
It's got to be some problem in the way the carb (or fuel injection) has been adjusted.
A sort of related example: my engine used to always shudder on shutdown. When it went to the USA recently (Lyco crank swap) they said it shouldn't do that; it wrong and is a fault in the fuel servo not fully closing off the fuel flow when mixture is on idle. They lapped the valve and now it's a lot better. Yet practically all UK pilots would have simply accepted it as it was.