maybe your pilot is to small or too big does it load up easy or have a hanging idle meaning the pilot is too small what is you air screw at 1.5 out from the closed pos. seems to work pretty well. it is effective between about a 1 to 2 1/2 then it is time to change your pilot. try a new plug. and always use fresh gas wheather it is from the pump or race fuel.
which one is the pilot ive tried to work on it and just cant get it is the air screw the plastic one or the metal one if to i got the metal one turned out about 1 1/2 or 2 turnsbut im not sure about the plastic one
I just had the same problem with my Yz250. i had to redo all the jetting. first off if your bike is breathing heavy your not getting enough air so switch to a smaller pilot jet. this will effect your 0 to 1/4 throttle so once you mess around with this it should idle. next let it idle and the shut it off and check the plug it should be a nice crisp brown if it is wet or black you are too rich so try raising the clip position on the needle to lean it out. if it is white do the exact opposite lower the clip position on the needle to richen it. this may take awhile so have patients. after you get it to idle properly and not to rich or lean ride it around and see how it pulls in 1/2 to 3/4 to full throttle if it pulls nice and smooth you probably have the right main jet. so after you ride it hard for a good 15 minutes stop and check the plug. if the plug is brown and crisp your main jet is correct. if its black and wet you need a smaller diameter main jet. if it is white or very light color go to a bigger main jet. remember this all takes time and patients so don't give up you can do it with a little bit of time and effort. also Marc Salvisberg has an excellent carb tunning page. google search him and go to his factory pro page. i know it's for bullet bikes but the same applies for dirt bikes the only difference is when he talks about the fuel mixture screw and how to get to this ignore it this is the air screw on regular keihin carbs. good luck.
How many times does this have to be said? 2 strokes can and will idle just fine. Your jetting is off. The 99rm125 has a ridiculously rich pilot circuit. With the 39mm PWK that's stock on that bike, it came with I believe a 55 pilot. When I had mine I went all the way down into the low 40s on the pilot to get it to idle clean. Once jetted correctly, it will have good bottom end, for a 125, idle all day, and never foul plugs.