As far as needle walk...the high speed needle has a ratchet clip like most engines so that shouldn't be walking.
If it's the idle screw, I did have that on a GMS, and an old worn 4BK carb on a FSR 40. The fix was to remove the idle needle valve and wrap the threads with teflon tape. You can tell by adjusting the idle screw if t's loose or not.

Most modelers out there have bitter / sweet experiences with Chinese engines when they expect it to perform out of the box and to be consistent as an OS, which is ONE the most user friendly engines out there, certainly the most popular. You can remove the needle stops as recommended, get help from an experienced Evo owner to help you tune it, or punt. Thunder Tiger 46 is cheaper than OS and has a loyal following. http://www3.towerhobbies.com/cgi-bin...&I=LXKF97&P=ML, $95 before coupons.

Before you spent $100, what fuel are you using, what prop, what plug? Have you tried changing these items first?

Every month or so the Evo story comes up, some complain of a problem, people reply with buy an OS.
http://www.rcuniverse.com/forum/m_95...tm.htm#9590808