Welcome crisspyT, Line up mark at idle on oil pump, Carb = back off cable till slide seats, screw back idle screw first tho, feel with finger on slide as opening throttle, then take up slack,[adjuster] then adjust idle, My DT400c had brass adjuster on top of carb, and long adjuster at bar end of cable,, what colour is yours , its 1980 since i sold my MX,,, sold my 400C in 2010,