main needle valve is adjusted like any other engine screwing it in will lean the engine,the low speed is set by the the barrel stop screw and the low speed mixture is set by the air bleed screw and yes screwing that srew in will richen the low end.usually start with the end of the scrw in the middle of the hole when looking through it and make 1/8 turn adjustments.start the engine ,set the high speed needle,then go to low throttle and set the barrel stop screw for the rpm you want,then you play with the air bleed screw for correct mixture in 1/8 turn movements.if the engine dies as soon as you go to low throtle it usually means too lean turn the screw in,if it idles and slowly slows and dies or hesitates going from low to high throttle with a lot of sputtering it is too rich and then unscrew the airbleed screw in 1/8 turn increments.you need to do this with the engine stopped for saftey reasons.