hi people i used to have a dtr125 and i had all the same probs .it was slow as fu*k .
so what to do now
1.in the expansion chamber on the exaust cut it and remove the restrictor.
2. while the exaust is off look down the exaust port and see where the power vave is. turn the ignition on if its sticking down after the ignition is turned on you need to turn it 180 degrese then ajust it with the cables so it sits flush with the exaust port turn ignition on and off again and make sure its flush.
3.remove snorkle from air box
4. on the inlet manafold the black rubber what the carb is atatcht to you need to remove that and when you look at it when its off there is a funny v shape bit on it there needs to be cut flush and and open the little hole up a little bit so it gets more oil.
5.and last but not least there is a little thing in the clocks next to 1 of the bulbs take it out and wrap it with black tape and just let it hang .
hope this helps
ps enjoy happy wheelying lol
or get a rd125lc engine and do a transplant or a 350lc engine and do a transplant lol