Loss of Speed and Revs



  • Hello guys,

    So here is the story: I bought my Dt125r 2002, 2 years ago, it was driving fast, quick response to 75 mph and i could redline the RPM.
    But it wasn't reliable, sometimes it bugged and acted strange.
    Than I thought, let's put it back original. I removed the carb, cleaned it and replaced the main jet (280) to original (240) the rest on the carb was original.
    I drove to work and on my way back it acted strange, it bogged and died..; (to lean) piston rings worn, so no compression.
    The previous owner, drilled holes in the airbox and cut the whole top open. (way to much air + main jet 240 = to lean.
    Took my Dt to the garage and the bored the cilinder out. 🙂 it was alive again.
    Now i fixed the airbox and putted a main jet 260 in it (scared to run lean again)
    But after the rebore, the bike isn't that fast anymore, maxspeed is 56 mph and max revs 9000 RPM. 😞

    Does anybody know what to do or what can be the problem.
    I would love that my DT could get his Speed and Revs back..

    Thank you. (Sorry for my english)



  • Stock airbox = stock mainjet, try that.



  • Yo, Welcome to the Forum

    Have you been running in the new Piston rings into your bore properly? If not you will ruin the new cylinder wall and loose compression again.

    Try a richer jet if it's bogging up top, like a 310- , need to determine if it is a lean or rich condtion,



  • @Jens-Eskildsen, this afternoon I put back the mainjet 240, maxspeed is 60 mph and max revs 9500 RPM.
    It's a little better, but not like it should be..



  • @Darty, thank you ;), I drove the x number miles calm and not to high rpm. But my mate got a compression tester, maybe I should test this first ?



  • @fets100 okay nice, mixture is probably in range. Definitely worth a compression check if your piston was the wrong size. But you should feel if it's low anyway.

    Check your exhaust for any leaks as this will be the main cul-prate for power-loss at high RPMs.

    Have a look at the Reed petals in the inlet manifold for any damage too, again will effect high RPMs considerably.

    Cylinder head damage? Worn clutch plates? worth a check.

    Good luck with it.



  • Yesterday I did the compression test 110 psi.
    alt text



  • hi
    i am new at forum. sorry for my english.
    i have the same problem 9000 rpm max. have you solved the problem on your bike. thanks



  • To the OP, was the compressiontest done with a warm engine and open throttle?