Carb what is the setting on air fuel mix - 2003 Yamaha YZ 125

sounds like it is starving for fuel mark. be sure the fuel lines are good and not soft mushy or hard. or cracked letting air get in . also if there is a fuel air mixture bolt you can turn that to set the air fuel mix to make it run better. if that don't work i would take the carb, off and clean it again. be sure the diaphragm is not hard. if it is replace with a carb, kit. replace the fuel filter also.

Probably fuel starvation.
Wrong fuel air mix settings on the carb.
Damaged carb diaphragm.

Probably both settings need attention. But you could have a sticking carb float too. Here is a pretty good procedure for setting air to fuel mix:

http://bikeadvice.in/tuning-carburetor-optimal-performance-fuel-efficiency/



If performance still lacks after adjusting dismantle and inspect carb.

to much air when mixing with fuel in carb find air mixtur screw on carb an whale running turn in slowley till it stops backfiring turning the screw in adjusts the air mixing with fuel the screw is useuly located on the side of the carb witch the toub hooked up to put fuel in carb is
or you could re set it by screwing all the way in then screwing out 3 whole turns then whale running turn out til it back fires then turn screw in haf to three quarters of a turn in hope this helps

The carb choke is a restriction in the air flow before the point where fuel is added to the air flow. When you set the choke, the throttle plate becomes the air restriction because the "mini" carb system feeds rich air/fuel mix in front (engine side) of the throttle plate. If you set the choke and open the throttle, the "choke" becomes ineffective...Hope this answers your question.

how many lines run into the carb or come out of the gas tank maybe you got them mixed up

do you have adjuster screw's on the carb???

if you do then screw them all the way in then back them out about 1 and half turn then slowly adjust until correct setting's are reached

If the gas is old, for example, last season, change the fuel ..drain the tank, carb and fuel line.
Also clean the air filter... this can have similar effects on start/run.

Pull the spark plug and clean it also, you can use spray carb cleaner on the tip, and then clean with fine sandpaper.
then replace the fuel with fresh fuel/oil mix.

A good spray can of carb clean will work to clean any build up in the carb jets, fuel line etc...

on fuel/air mixtire you turn it all the way in till it stops then back out 1 1/2 turn to set air / fuel mix

Try disconnecting the mass air flow sensor on your air intake. If it runs better after you disconnect it...it's bad and needs to be replaced.