Motorcycle Repair: carburator, air screw, air leaks

Question
QUESTION: Hello i have a 2007 baja 125 cc dirt bike i'm told they are Pantara Chinese build bikes.My carburator will not idle
propely or when i come to a stop the idle will have wide variations in the rpm withought toching the trottle.also when i am ridding and let go of the gas the rpms stay high and all this happens only when the engine is at operating temp.Can you help thanks

ANSWER: Eric,

-have you made sure the cable has some free play
and is not sticking?
-I am not sure what engine you have, some
of the Baja's had governor systems on the carbs
which open the throttle under load.
-If you have a standard motorcycle type carb
I would check that the throttle slide is
not sticking, also look for any air leaks
around the carb mounting flange. Spray some
carb cleaner around the flange and see if engine
speed changes.
Check that you have a good flow of fuel
to the carb and the carb is clean.


---------- FOLLOW-UP ----------

QUESTION: Hi wayne it's not the throttle sticking no air leaks good fuel flow my engine is a 4 stroke cdi ignition,standard carb,sheng wey japan.New bike so carb is clean,my spark plug is black and sooty is this caused by a too rich of a
fuel/air mix and can you tell me turning the fuel/air screw clockwise does what and counterclockwise witch is lean and rich mix is achived.I'm out of ideas can you help.
thank you

Answer
Eric,

if your spark plug is black and sooty,

-your choke is on,

-your air filter is plugged up or too oily if a foam type.

-Possibly a sticky float needle valve in the carb causing
it to flood the engine with fuel.

-carb jet has come loose.

Usually smaller carbs have an air screw, clockwise
for rich fuel/less air and counterclockwise for
lean fuel/ more air.
Check the air filter/choke and carb jets
before the fuel/air screw.