bought a 96 dt125r in very good condition 5,500 miles 2 owners etc etc.previous owner didnt have much of a clue.i took it for a ride in a built up area thinkin mmmm seems a bit slow but decided to buy it anyway because of the condition
bought a powervalve kit for it (servo,cables etc)
battery wasnt charging so bought a new one and still didnt charge,turned out the reg/rec wasnt knackered so replaced that,hey presto prob solved
still left with the underpower problems though.engine is good never been messed about with,ticks over and starts perfect,everything standard.
i have riden many restricted tzr and dt's before and this one is struggling to top 45mph! locked the powervalve open,checked exhaust port for correct position and just wouldnt pull past 7,000 revs! took it to my local bike shop who ive known for years and have dealt with 100's of dt's in the past they fitted the powervalve,all done correctly and the pv works fine and operates as it should but still no power!top speed is only 50 mph!! and is v slow,pulls like a 50cc! theyve cleaned the carb,tried different jetting all to no avail,just no success! there at a loss to what the problem is and everyone is confused,electrics havnt been messed about with,original cdi is on,no mods etc bloody frustrating,its been in the bike shop 5 wks now!! PLEASE PLEASE ANY SUGGESTIONS ps its a uk model and the front pipe has no restrictions ie restriction sleeve or internal restrictions,ive split the exhaust and had it re-welded just incase!
i think i know what your problem might be, mine was just like that until 3 days ago when i cleaned the exhaust with caustic soda, it made a difference but not a big difference , still can't get my bike over 60 mph though but mine is fully restricted and the exhaust had chunks of carbon falling out of it and i have heard about bikes not reving higher than 6 k rpm bc of clogged exhausts so try doing that.
i noticed that my bike runs better but it still pulls like a de-restricted 50 cc =(
i have the exact problem you have. get under the seat, check if there is a lil air snorkel looks like .. https://i881.photobucket.com/albums/ac14/craigbulmer/DSC00044.jpg
that will then let you revv past 8k, but past that i have no idea m8, i top at 55mph and mines derestricted, powervalve works properly if you find out why please let me know!!
cheers for confirming it may be a weak spark,only the coil left to check,all the wiring is good and tried 2 cdi's!! still the same so a new coil should be here tomorrow and will let you know then,do you know if the coils do have a habit of failing/playing up on dt's? it did need a new reg/rec when i bought it as the battery wasnt charging,also replaced the battery just incase,cheers
i would look at the set up of the powervalve, as i posted b4 my problem was like your, it turns out the valve was only opening 2/3 maybe 3/4 and now i set it up (its really easy aswell!) and now it flys!! not sure what the top end is but ive had 70 :D its an unbelievable change, worked for me and it doesnt take long :)
Hi did the coil fix this problem as im having same problem
cleaned carb filter new plug another cdi reeds pv set up any way you like
Just sound diffr
Just sounds different , and when you go into the power it just splutters and some times kick in .
My old aerox did the same thing as this , it was the coil , but then my yz85 05 , did the same thing and when i turned the petrol of it would fly just like the dtr is doing , its driving me crazy. many thanks
and i have heard about bikes not reving higher than 6 k rpm bc of clogged exhausts so try doing that. 
disconnect,peg open and you'll know 
