Hi riverrat,
You may have already done this, but if so it may be worth doing again.
Use compressed air or, at a pinch, WD40, blow through the orifices at the inlet side and also the air/mixture screw hole.
If this does not help, remove the carb and dis-assemble very carefully, taking particular care not to damage parts.
Check that parts such as jets. slide, needle and mixture screws are standard, using your other carb and your "shop manual" as reference.
Blow through all orifices using compressed air or WD40, although this leaves a film of oil but does not usually cause problems.
It is often well worthwhile immersing the carb in carb cleaner for a day or two, or at least a couple of hours.
Check the float for air leaks by immersing it in hot water. Bubbles will indicate a hole or holes. It is possible to repair these using a soldering iron and solder. Mark the hole/s, cool the float in the fridge (not freezer), and solder the hole/s using a minimum of heat and solder.
Set float height to the standard setting and re-assemble, taking care not to overtighten jets and screws and setting the needle position and air/mixture screws to the settings described in the manual or as seen on the other carb which works ok.
A useful tool for checking float height in operation can be made using a spare drain screw or something with the same thread drilled down its centre, soldered or brazed to a short piece of tubing to accept a piece of clear plastic tubing so the fuel level can be seen when this is held against the outside of the carb with a cable tie. Leave the other end of the clear tubing open to atmosphere. You could make two, and compare both carbs at once.
You may already know all this, in which case I apollogise, but if not, hopefully this will help sort your problem out.
Good luck.
A.
