2007 yamaha ttr 125 wont go into gear. starts fine. try to shift no matter if you try first or second or any gear it won't go into gear just idles and wont move

I agree it is a broken tooth on second gear in the trasmission. You will need the trans. rebuilt before riding further or you will jam the transmission, Break more parts, & possibly break the main engine cases. DO NOT RIDE !

sounds like the gears inside are stripped or missing teeth open up the side case and check and replace any gears missing teeth or stripped

The bike has a bent shift fork. The engine/transmission must be totally torn down to get to the shift fork. This is not an easy repair and some special tools are needed. I do not recommend the repair be attempted by the home mechanic. Call around to dealers for quotes on the repair. It will cost at least $750 I am sure. Sorry...

Time for carb cleaning, blow the jets with compressed air and check the fuel level in the float chamber, adjust if necessary.
Tht factory plug is where the air-mixture screw is and tht plug has been kept there for non tampering!
That screw is factory set.
Basically its the primary jet which is a problem in your bike's carb, but all in all might as well blow both of them.
Hope this helps!

You have a broken tooth on the first gear set. This means two gears will need replaced; the drive gear and the driven gear in first gear range. DON'T just replace the gear with the broken tooth. Replace the gear it meshes with also.