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  1. #1

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    genuine thumpstar 125 gearbox probs

    hi,
    my gearbox is very hard to shift from 1st gear to netral/2nd gear.
    is it worth tearing the engine apart to try and find the problem (bent shift fork i presume)?
    how much work is involved?

  2. #2

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    full tear down, head/cylinder and splitting cases required!

  3. #3

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    i feared so :-/
    and then there's the problem of finding the correct pieces, since there's so many versions of these engines...
    or can i just find them by using the engine number : 156FMI...?

  4. #4
    motodevo's Avatar
    motodevo is offline Miniriders OG Master Spanner Spinner
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    If its got a 1N234 shift pattern then its a common issue, thats why most engines are now all up (N1234). There was a improved shift star available a few years ago from the US (SDG from memory) and swapping out the shift arm spring which improved the problem. I would take the clutch cover off and check your shift star is still torqued tight and your shift arm hasn't jumped the pins on the shift star and is sitting behind where it should be, i had that happen after i layed it down on the gear lever sidePicture528.jpg
    Last edited by motodevo; 08-09-2017 at 08:12 PM.
    Too many zeds in the shed

  5. #5

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    allright, will do that.
    i'll post back what i've found.

  6. #6

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    so what i've foud was the shape of the shift star that was making it hard to go from 1st to neutral.
    because i wanted to fix this instead of waiting and order that SDG shift star, i got to work with the dremel :-p
    it now shifts pretty much ok, not perfect but doable :-)
    the roller arm that holds the shift star in place was pushing too hard on the star + the shape of the star made it too hard.
    glad i didn't have to open up the engine! thanks to @motodevo for the golden tip :-)
    now i can move on to setting that bike up right.

    cheers, Hannes.

  7. #7

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    are these thumpstar bikes chinese of japs? or do thupstars come with different kind of engines/quality?
    is it worth upgrading like bigger carb, 500gr flywheel?

  8. #8
    motodevo's Avatar
    motodevo is offline Miniriders OG Master Spanner Spinner
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    They are chinese, they were the better quality chinese bikes in their day, but pretty much all use the same engines. Carb wise, dont go no bigger than a 26mm mikuni knock off for a 125 (most 125s run these anyway). I wouldn't waste any money on a lighter flywheel. Best bang for buck upgrade is a bigger motor, chinese motors are nice and cheap
    Too many zeds in the shed

  9. #9

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    i've got a lighter flywheel laying around on another engine i bought for pieces.
    is a lighter flywheel better or worse for overall performance on these engines.
    this thumpstar 125 has significantly more power/torque than my other lifan 125 engine...

    too bad that other engine's too much damaged, con rod bent, cilinderhead prettymuch destroyed from a broken off valve. it's a yx 140, rest of the bottom end should be fine.

    i might get a yx 160 of all else is upto spec (suspension-wise)

  10. #10

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    will a yx 160 engine even fit this lil' frame?
    will it bolt right up or need to reweld/ adjust hangpoint for the engine?

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