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So I have a yzf r125 it had broken down on me last week Sunday after a long journey (checked oil levels prior to going out on it) I was told it could possibly be the starter motor and to let a bike mechanic have a look apon further investigation from them they told me my engine had no oil. Thought it was strange as I had checked it. Anyway decided to take the bike back from them and top up some more oil anyway now the starter motor is just cranking like it’s trying to start is this classed as a engine problem or actually the starter motor?
 

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2008 YZF-R125 (180cc)
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if the engine ran with no oil, even for a short time then there's a good chance your cylinder is scored and not making compression anymore.

If that's the case then when you crank the starter it will turn over very freely, like you can tell it's not having to make compression. If you want to double check this by hand you can pop the little plastic cover off the centre of the magneto cover (the left engine cover) and stick a socket in there and turn the engine over by hand ( i think anti-clockwise is how the engien runs).

If it doesn't get more difficult to turn it on some parts (compression stroke for exmaple) then your rings/cyclinder are probably fried.

This is kinda what that looks and sounds like.. note how once i've stopped and am trying to start it the starter is just spinning really easily. If yours is doing that then that might be what's wrong.The videos after that one show what that looked like inside and how it got fixed..

(engine goes at 12:27 if it doesn't start at the right time)

Don't be tempted to go for a big bore kit unless you're willing to go down quite an expensive rabbit-hole of fuel controllers, o2 sensors and endless tuning, but whilst a straight cylinder swap is still quite a complex job, it can be done at home if you have some decent tools and are reasonably mechanically minded (or have a friend who is who can help).
 
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