ORIGINAL: Flying Bougie
The engine is a Magnum XL 25A. It only has the one main needle valve, and 1 screw (for air bleed). I've never touched that screw yet.
Either your engine is not a XL25A, or you have not seen the low end valve.
The low end should be adjusted by you.
http://home.wanadoo.nl/pereivers/engines1.html
If you don't have the manual for the engine, this one will serve you as a guide for manufacturer's recommendations for break-in and further adjustment:
http://media.hobbypeople.net/manual/210756.pdf