yamaha 125 starts but dont run right

[RubberSalt]

Alright, check these things; carb/air filter, reeds, air leaks, recoil starter is wound correctly.

If it doesn't go past 1/4th throttle, then odds are that it's a fuel issue. Also mentioned that your smoking a ton...
Change the needle clip position on the carb. Try the middle spot or the 2nd notch from the top. You might also have a main jet that is just way to big, try needle first. Make sure your air filter is clean, a dirty filter restricts air flow and creates a larger vacuum in the carb. carb + stronger vacuum=more gas leaving carb.

Recoil starter; If this is wound the wrong direction, when you go to start it, it will fire very early. There wont be much compression before ignition meaning a weak fire. Youll get smoke and thats about it... when you push it, it runs - thats a sign of a recoil wrong backwards.

It is possible that the reeds are not fully closing, this can really hurt things when it comes to starting - take the air box off and put your hand behind the carb after you get it running. If you feel tons of gas on your hand.. or even see it coming out(like i did with mine) then you have leaky reeds. This causes excessive smoke when running and hard starting.

air leaks; could have a bad seal, causing excessive air to leak in. The previous own could have tried and compensate with more gas but never got it right so he sold it.. More gas+leak=never going to run right.

I put my money on carb/filter and recoil.

I run a 32:1 in my 175 and my 125 - they like that number with some good oil (i know i could go leaner but im paranoid)

[yamaha driver]

ok for rider not being mean just saying i wear a helmet. if i ride in the ditch i get b%(^hed at because they bale it around here for hay.. all the fields have corn in or beans so i cant go there... i have no trails around at all except private... all my neighbors ride dirtbikes/ atvs everythin on the road BTW im 10 miles from a little town so im in country.
ive got holared at so many times for driving anywhere except the road so i asked if my noisy trikes (300r) bother any of my neighbors and none care if i drive anything on the road. and rubber salt thanks for the info but i swaped reeds from a running 125 same thing happened. tried all needle postions notin... the main jet in there ran great the same day ... cleaned tank i made sure no water in anything... we wound correctly i made sure of that and i still have oil injector on mine with fresh 2 stroke injection oil. left aircleaner on ran bad took off completley right from carb ran excact same.. when i put my hand behind the carb i feel a little gas not much. i shot starting fluid around boots and head... all gaskets never ran diffrent (P.S. i did it when it was running).... im so confuzzled... thanks for posting

[just ben]

Recoil starter; If this is wound the wrong direction, when you go to start it, it will fire very early. There wont be much compression before ignition meaning a weak fire. Youll get smoke and thats about it... when you push it, it runs - thats a sign of a recoil wrong backwards

If the recoil were wound backwards it wouldnt work at all. the dawgs in the recoil only work in one direction. and if the engine were turned backward it would fire late not early, if affected at all. depending on the particular engine.I have run many mx bikes and snowmobiles "backwards" just to show that it will

[RubberSalt]

check your plug, see what it says

Still sounds like carb has something plugging it up some what. Tiny something in the main jet or the choke is stuck on. Check the choke isn't on after starting. That choke might be out of adjustment, or cable is busted somewhat.