I have a Yamaha yz 125 dirt bike that wont start. Why?

Question

The rear wheel locked while riding, and then the engine shut off. There is compression when I kick start it and it sounds like it wants to start but it does not go all the way. What could this be from?

Answer 1

Best Answer - Chosen by Asker

When the rear wheel locked up, the piston may have seized to the cylinder.
When the engine cooled down, it may have un-froze itself.
With a scored piston and cylinder, there wouldn't be enough compression to start the engine.
2 stroke engines can give false compression readings.
It may feel like it has compression, but just that tiny amount of compression that blows past the rings, could be enough to prevent it from running.
Remove the exhaust pipe and look in the exhaust port with a flash light and mirror.
If you see any scoring on the piston and cylinder wall, or discoloration below the rings, the top end will need to be repaired.
If all is good -
Check for spark
Clean the carb

Asker's Rating:

Asker's Comment:

it was the piston that seized. any clue where i can find a chead pisten sleave and piston?

Answer 2

Got gas?

Battery charged?

Is there oil, but not overfilled?

Do you have gas flow at the carburetor when you take the hose off the inlet?

Is the sparkplug clean and the insulator not cracked?

Do you have the run switch turned to ON?

Man, it could be a hundred things... load it up in a pickup and bring it over and I'll look at it for ya.





g-day!

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Answer 3

new spark plug

Answer 4

well it looks like everybody has an answer for your problem. Confused yet? You should be. Anytime the rear end locks up your cause could be a book report of why... How long have you owned the bike ? Is it new to you? DID you not know that this bike runs on premix fuel? You may have seized the piston and wiped-out the cylinder. If so you need to have the top-end over hulled. You can get yourself a Manuel on this bike and preform a compression test. The how and why will be in the book like a dry and wet test to determine what is wrong. You may also had a revalue bake and lodge it's self in the bore and locked up the piston and, then when you tried to restart it you may have kicked it in to the exhaust header or, the internal part of the spark plug fell into the bore> I myself had seen this when someone new to cycle mechanic is trying to work on there bikes. they don't know how to gap the plug right or, they drop it on the ground and next thing you know you have a problem.
If you can check out the basics on ""the"" how to and, can't get it to run. You will save yourself a head ake if to take it in for a service check. You could find more bad things starting to happen and cost to much. Bring it to your local shop make some friend while your at it you may find some new ridding buds to go out with on the week end and have more fun then whats wrong with my bike...

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Answer 5

For sure, it sounds like top end issue. I had a KX 125 that did the same thing. In my case, I knew there was catastrophic damage in the top end. When I would kick it over, I could head a lot of mechanical 'ping' sounds. As the top end was removed, I saw the hole along the exhaust ports. Rather than trying to reacquire a costly cylinder head, I traded from a junker car. All things told, I wish I kept the bike :(

Keep in mind that the compression reading isn't a reliable gauge for you to work with in diagnosing a locked up rear wheel. Chances are very good something happened in your top end to have caused the lock up in back. You'll need to remove the top end to make an inspection. If you're fortunate, there's not a lot of mechanical damage inside. Chances are, though, that some kind of damage occurred.

Aside from your top end, check your spark output from the plug by kicking over the bike a few times with the plug touching the bike's motor; you should see a whitish-blue spark. And if the bike was running fine up until the lockup ,chances are the fuel delivery system was sound. You can look inside the carb's float bowl, regardless to confirm any doubts.

Answer 6

Sounds like a seizure to me,possibly rings smeared.In some cases there is compression but just not enough to fire up