There should actually be 3 adj. screws. A high speed needle, a low speed and an idle screw. The main one is the most obvious. The idle usually comes out of the front of the engine a bit, and the low speed is usually located on the center side of the engine. Some combine the low and high speed needles together.

There's actually a great article on redcat racing about tuning that describes them pretty clearly (and I guess it may help that it's for the same or similar cars).

http://www.redcatracing.com/mod_details.cfm?mod_id=12