The procedure for adjusting the carburetor on your Fox .15 BB depends on which carb it has. Go to the site Doug recommended but use this link:

http://www.flitelinesolutions.com/carbs.html

Look at the photos of carburetor's and determine if your engine has the "Early 2-jet carburetor" or the "MKX Series Carburetor". Either one could be on your engine (depending on when it was manufactured).

If you have the "Early" carb, the procedure is exactly opposite from the way most carbs are adjusted - you must adjust the LOW SPEED NEEDLE FIRST.

Fox engines are good engines - if you follow proper procedure. A lot of folks try (tried) to adjust their carbs like other engines, then when they couldn't get them to run right, declared them "junk".