You don't mention if it runs better when warmed up or if you have to prime it to get it to fire. My experience with the Weber is that vaccum and or seal leaks become an issue. If you run this item without an air filter, you have opened up the Pandora's box of possibilities for restricted air passages and dirty jets. This forum is primarily for fuel injected late model Ford Fiesta sold in North America. You might still be able to download the service manual thru Wikipedia.
The linky to that is IH8MUD forum/Weber carb adjustment. Or something like that. It's been a while since I have been there.