ORIGINAL: LNEWQBAN
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
Please read carefully the break-in procedure specified in the manual supplied to you by LNEWQBAN. VERY similar to OS, the procedure involves starting up then advancing to FULL THROTTLE in a very rich condition then slowly leaning out the high speed needle. I suspect this is not how you have been proceeding.
Once the engine runs at full throttle at high needle settings from rich to lean, set that needle at a speed a little below top rpm and THEN adjust the low speed needle. Finally, REadjust the high speed and you should be ok.
Best of luck!