dt125r carb
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@dtr-mkd Pre 1991' or so, run a Mikuni VM26ss which is a roundslide design.
After that all DTR/RE/X models use a Yamaha Mikuni TM28ss - Flatslide.
The VM26 is restricting, but good for low end torque, problem with roundslide carbs is that the design disrupts air flow considerably which slows the intake down.
For a standard DTR, look for a Mikuni TM30ss, used on the Yamaha WR200. probably the perfect carb upgrade for a 125R.
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You can also try a Delorto VHSA32 which is a 32mm carb used on the TZR 4DL which shares the same engine as the DT125R.
Only putting it out there, if it were me, I'd go for a tm28ss which is standard on DT125R from 1991 as Darty says.
The jetting changed in around 1996 as well but that will depend on the conditions of where you ride so you'll have to experiment with that.