[Darren69]

Originally Posted by Mr Gazza

This could be a float height/operation issue, or perforated diaphragm.

Another thing to check would be the triangular grey plastic boxes that sit in the frame either side. It is important that these are present and that the relevant pipes are correctly placed in them and are in good condition, free of splits and leaks.
The boxes provide a place to draw still air from for the pipes, but I'm damned if I can remember what those pipes do and why they need still air except that if they don't it will run rough, though probably more at higher speeds than slow and definitely worse in windy weather.

Sounds like an Italian bike, it depends on the weather