To get the proper fuel-air mixture there are three systems in the carb to consider:
(1) Idle system – this controls the mixture at idle to about 1/8th throttle.
(2) Needle jet – this the needle you see in the carb in combination with the needle jet. From about 1/4-3/4 throttle
(3) Main jet – this is the throttle wide open.
To adjust the idle system
alternate between adjusting the idle speed screw to the lowest RPM and the idle mixture screw to get the highest RPM
For the KTM 450 EXC-R:
(1) turn in the idle mixture screw till it stops. Unscrew it 1.75 turns
(2) run the engine 5 minutes, until warm
(3) turn off the choke
(4) adjust the idle speed to 1600RPM
(5) turn the idle mixture screw clockwise until the idle begins to fall
(6) note the position. Turn the idle mixture screw counter-clockwise until the speed begins to fall
(7) adjust the idle mixture screw between these two positions with the highest RPM
(8) re-adjust the idle speed screw to 1600RPM
To adjust the needle system
ride the bike at 1/2 throttle up a hill for about a minute. Then quickly pull in the clutch, turn off the key and pull over. Remove the spark plug. White plug, move the needle clip down. Black sooty plug, move the clip up. Tanned plug (ready for the beach) – leave it alone.
To adjust the main jet
Ride the bike full throttle up a hill for a minute. Then quickly pull in the clutch, turn off the key and pull over. Remove the spark plug. White plug – bigger jet. Black sooty plug, smaller jet. Tanned plug (ready for the beach) – leave it alone.