97 kx125 won't start

Alright my bud just bought a 97 kx125 and it ran fine when he first picked it up, absolutely no bogging. When we get back and started it up it started bogging when riding in first gear at about 1/4 throttle. He shut it off and we were going to see what the problem was a couple days later. Just for kicks it started up and everything seemed fine, he rode for about 45 minutes and then just cut out. It wouldn't kick over. We replaced the spark plug and it is getting spark. Air filter is clean, we poured about a teaspoon of premix down the spark plug hole and back fired once. Tried bump starting it in 2nd n still nothing. Took it apart and seems like its getting gas flow thru the carb, the reeds look good (were a little stiff, thought they should flex a little), there was also a small metal chunk behind the reeds which makes me wonder how the cylinder and piston are. The owner also said he replaced the piston and was partially broken in, wouldn't think 45 mins of casual riding would cause this problem. Any info or sugguestions would be appreciated.

Pull off the exhaust and look at the piston on the exhaust side. See if there's any mushing of the piston material over the rings.

Piston and rings look brand new. Also, before we took it apart and tried bump starting it in 2nd, the bike could push in gear without the clutch engaged almost as easy as if it were to be in neutral.

bump

How hard is it to kick over? Does it feel like there is compression when kicking it?

Check the woodruff key on the flywheel isn't sheared. I completely rebuilt my kx125 and didn't recheck the flywheel bolt. It came loose and the key sheared. It would only backfire every now and then when I kicked it over.

I've heard of others having the same problem and have a good hunch that the woodruff key is sheared, the previous owner must of forgot to tighten the flywheel bolt after he put in a new piston. I'll check the woodruff key but I have never took off a topend before, any tips would be appreciated. I have a general idea and some diagrams but I know these can be tricky.

Generally, the flywheel would not be taken off during top end work.

Don't need to take off the top end. Just remove the stator cover next to the shifter and it is in there. It takes less than 5 mins.

Don't need to take off the top end. Just remove the stator cover next to the shifter and it is in there. It takes less than 5 mins.

KDX has a good point. Why would the previous owner be taking apart the flywheel to replace the top end? Most likely not the flywheel....

KDX has a good point. Why would the previous owner be taking apart the flywheel to replace the top end? Most likely not the flywheel....

Doesn't mean it it hasn't happened. Like the people who take their car in to get the brakes fixed and a week later the transmission fails. Totally unrelated but machines fail. It is an easy check and something I'd at least look at before tearing into the top end.

Is there any other way of getting the flywheel off without using a flywheel puller?

Found the problem. The woodruff key is fine, however, two of the wires connecting to the stator broke off. If anyone has one for sale or are parting out please let me know. thanks

Sounds like piston to me .

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