What engine is it , if it's a zetec then I think if I remember right they have a little brass screw next to the lever spring with silicon or paint over it , that should be set from factory and the idle speed control valve should do the rest. Take it off soak with carb spray inside for 30 mins clean and empty and refit . The gasket will probably be fucked so refit with silicon but let it dry before running. See how you get on , I used to clean mine every 6 months as I would start to hunt its bacause it reburns oil from the breather for emissions and just coaks everything up.

cheers paul