genuine thumpstar 125 gearbox probs

[user 20441]

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?

 

[AntNZ]

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

 

[user 20441]

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...?

 

[motodevo]

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 side

 

[user 20441]

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

 

[user 20441]

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
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.

 

[user 20441]

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?

 

[motodevo]

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

 

[user 20441]

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)

 

[user 20441]

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

 

[gottagetacheaperhobby]

Light weight flywheel
Depends in what you mean there, the inner rotor kits are rather hit and miss from China, and can leave your crank unbalanced, which leads to very nasty snapped cranks. The lifan 150 adjustable stator or the zongshen 160ho stator are lighter than a stock 125 stator, but I'm not sure if they fit or not (never really played much with the lifan 125)
Lighter flywheel does equal better response, and higher rpm, but at the cost of torque, hence the flywheel weights you can get for 2 strokes, a naturally low on torque engine.
Engine swap
Unsure on the exact frame your using, but I squeezed a 160ho into a crf50 based motovert frame no worries at all.
Another option for engine is the gpx125 that dhz sell, hi comp square engine that revs to the moon and has an amazing punch for a 125, with the advantage of running a stronger bottom end than most other 125s. I've owned one, and apart from not having as much torque as the 160s its a pretty close comparison to them. In a light bike, it's the perfect engine IMO.

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[rotn50]

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)

Lightened fly wheels (irk) aren't worth it. And on a start in neutral only bike (clutch on the crank) I have seen them destroy the clutch into thousands of pieces and also destroy the clutch case.

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[user 20441]

nt an IRK but half the weight flywheel. don't want to loose too much torque.
don't like the fact that the crank spins unbalanced without any FWW.

 

[user 20441]

might aswell continue in this topic since it's my own on this genuine thumpstar
i was wondering what tyres to fit on these 12" front and 10" rear wheels.
i plan on riding all kinds of soil,including some gravel maybe, so hard knobbies will apply.
please add size of the tyres too so i'll know exactly what to look for.
current tyres are no good anyway for what i do.
the back tyre will need to be wider.
front i want sharp knobbies to grip in grass, mud...
and while i'm at it :-D what sprocket setup is advised for offroad? don''t know what current is, but all is stock.

this is the bike i'll be fitting them to.
later on i might be upgrading the engine with a yx 160 or whatever is advised/available in the same price range...

 

[gottagetacheaperhobby]

I built a motovert frame (same size as yours) with a 160ho, was an animal
Kenda washagual are the chunkiest 10 inch you can get i think, I ran a kenda milliville front, but can't remember the sizes sorry.

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[rotn50]

The gears still playing up?

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[user 20441]

all good now, worked the shift star myself,shifts nicely now, just a lil' harder to find neutral because of what i did with that shift star.
one notch was too steep to go from 1st to neutral ,so i rounded that one off a bit to meke it less steep, so less pressure was needed to shift up from 1st to neutral.

 

[rotn50]

all good now, worked the shift star myself,shifts nicely now, just a lil' harder to find neutral because of what i did with that shift star.
one notch was too steep to go from 1st to neutral ,so i rounded that one off a bit to meke it less steep, so less pressure was needed to shift up from 1st to neutral.

Excellent. Also the lever with a hook either end that pushes the star gear can need removing and with a small round file make the hooks a tad sharper as sometimes the ends go blunt and don't push the star pins enough.

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