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autobahnyea

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I bought this 125cc pit bike (chinese) i got it so it ran. Rode it for 100' and could only hear exhaust leaks. Fixed those, then rode again and heard a real bad noise from the trans. Shifting was hard but do-able. Rode for 50' then rear tire locked up. Now it wont shift at all. Googled and no results. I don't want to take side case off without knowing what to look for. Any ideas? Even the adjustment screw on side does nothing to help.
 

motorman

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I bought this 125cc pit bike (chinese) i got it so it ran. Rode it for 100' and could only hear exhaust leaks. Fixed those, then rode again and heard a real bad noise from the trans. Shifting was hard but do-able. Rode for 50' then rear tire locked up. Now it wont shift at all. Googled and no results. I don't want to take side case off without knowing what to look for. Any ideas? Even the adjustment screw on side does nothing to help.
Could be a couple of things.

Sounds like the gearbox, the shift selector drum sounds like it's jammed.

Could also be the top end that has blow and locked the piston or rod inside the bore.

Only way to tell is to strip it down. If it's only a 125 might just be easier to source another bottom end or whole motor.
 

autobahnyea

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the motor will still turn over by hand. one thing I noticed before it completely locked the back brake up, is when rolling it in neutral, the back wheel would lock up momentarily then release.
 

autobahnyea

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So, if I take the right side cover off, where will this selector drum be, and how would I tell if it's jammed or in need of replacing. Also, the bike has a semi-automatic clutch
 

motorman

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So, if I take the right side cover off, where will this selector drum be, and how would I tell if it's jammed or in need of replacing. Also, the bike has a semi-automatic clutch
Not quite that easy.

You need to split the engine cases.

When you open the cases you will see straight away what the issue is.

Don't as me how to split cases, that's were my mechanical knowledge stops. lol. Have a search on the site though as guys have done it before.

Have you got any details of the engine? Model No or something?
 

motodevo

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Above the sprocket on the motor, there should a black rubber grommet/disc about 20mm in diameter. Pop it out with a small flat blade screw driver. Once its out you will see a 10mm bolt head. Put a socket on it and see if you can spin it, this is on the other end of the shift drum. Sounds like you bought someone else's problems. These engines are pretty simple once you know them. Jump on youtube and start learning. Try searching for Alex Mann, he has done alot of vids building and rebuilding and building up his honda z50r, internally just about all auto clutch pittys are the same as a honda z50,xr50,crf50, etc. Once you have opened em up a few times it all makes sense.
I reckon you have a few problems. First you said it was hard to put into gear, could just be clutch adjustment. But then you said it locked the rear wheel makes me think its inbetween gears, possibly the shift star screw/bolt has backed out or the shift arm off the shift shaft has moved/jumped out of its groove between the shift drum and shift star. Take off the Kickstart, thrn the clutch side cover and jave a look. You will see where the gear lever shaft comes through the cases from the other side, there will be an arm coming off it going to the shift star/drum at the top of the motor above the kickstart. There is another splined arm that goes to the clutch to engage it on gear changes. Any of these issues the minimum you will need to do is remove the clutch, gear lever and the large gear next to/driven off the clutch. You will need a manual impact driver,clutch nut tool and circlip pliers.
Only way to find out is to rip off that cover and take a peep. Watch some youtube vids, like i said once you get your head around these motors you can build them in your sleep. If you aren't sure about something take a pic, someone will chime in with the answer or solution
 
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motodevo

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Okay, so a pictures worth 1000 words so i went out to the shed and yanked the cover off an auto 110 i am fixing up, figured it would make more sense with a pic(plus i had to get away from the missus and her junk reality TV show) You can see the shift star is hiding in the top right corner behind the big gear driven off the clutch

IMG_20170301_23718.jpg
 

autobahnyea

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Okay, I began disassembly and have some questions. Firstly, I found the big gear has a couple teeth busted off of it. Could this be the cause of my problems or just something else that's wrong? Also there are these tabs and one of them is bent. Not sure if this is correct or not (shown in picture). Also how do I get the clutch mechanism off. Is there a special tool? Can't seem to find anything about that on google.20170303_171927.jpg20170303_171939.jpg
 

autobahnyea

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More research shows that is a lock washer that's supposed to be bent, and that you are supposed to have a special tool. Is tool required or is there a way around it?
 

motodevo

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Yep lock washer is supposed to be bent to stop it backing off when running. You need a clutch nut tool, google pit bike clutch nut tool, once you see it you can order one/buy one for like10 bucks off ebay, or make one out of an old socket cut down with a grinder to fit the nut. If you are really desperate/dodgy you can smack it loose with a flat blade or chisel in one of the notches in the clutch nut (not recommended but plenty have done it before, myself included).

IMG_20170304_759.jpg

If you wanna play with pitty motors I'd recommend buy a few essential tools. A clutch nut tool, a flywheel puller, an impact driver with bits and circlip pliers.

Though you need to replace that gear or else your just wasting your time, and mske sure you find the broken teeth, if you don't they will probably end up lunching your gearbox next ride!
Aren't pit bikes fun!?!
 
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autobahnyea

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Okay so the tool came in the mail today so I could get it farther apart. I got the large gear of so i can see the star gear and all that. If i mess with it (jiggle it around some), sometimes the bike will shift but only have like 2 gears. The screw holding the star piece on seems to be pretty snug. Now what? I don't want to dig deeper without an idea of what im looking for. Thanks everyone
 

Luke.note9

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Hey m8 did u find out problem my bike is doing the same,I've torn it down nd put back together like 4 times I'm thinking of sending it to the shop,I can't find the problem.
 

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