YZ125 Help

Hi all. I'm in need of some help. I've read and researched so much already, hoping that there's someone out there that can offer another repair option/advice.

We have a 2002 yz125 that will not start. It will fire for a fraction of a second on perhaps 1 out if every 8-10 kicks, but that's about it.

I've tried (not necessarily in order) the following: checked compression, currently at 150, airbox, filter, and

vent lines are spotless, cleaned the carb, then rebuilt the carb, and cleaned it again later. Drained tank, checked for gas flow. Used fresh gas even though we go through 20 litres a week between three bikes. Checked for Spark, looks good. Aerated the engine thinking it might have been loading up. Tried bump starting it multiple times. Removed the cylinder head to check for carbon buildup, but was fine.

Any suggestions?

Reed petals sealing good still

I think you missed one of the easiest things of all--CHANGE--not clean, the spark plug.  Fouled plugs are very difficult to clean effectively,and while they may show a spark after a cleaning, they foul immediately when trying to start the bike.  Go down to your local auto parts store and buy a couple of NGK BR9ES plugs for about $2 each.  I bet that get's it running...

Forgot to mention I replaced both the plug and reeds.

Hi all - still hoping for some suggestions or insight on this. Anyone???

Check your ground connections on the frame and ohm out the stator and coil.  If you haven't already.  That would rule any of that out.

I'll check that tonight, thanks. Just to clarify though, can there still be spark even if the stator was out of spec and/or there was a poor ground?

I'll check that tonight, thanks. Just to clarify though, can there still be spark even if the stator was out of spec and/or there was a poor ground?


While I haven't come across that scenario myself yet, I would think it could spark with a poor ground.  Just not a strong enough spark to ignite consistently.  Maybe someone else can pitch in and elaborate on this point.

It takes a lot of voltage to cause a plug to spark.  The higher the pressure, the more voltage it takes to jump that gap.  With the plug out, you're at 14.7psi/1 atmosphere.  It is very possible for a plug to be able to spark at atmospheric pressure but fail to spark at 100+psi of compression.  I've run into this before - you see spark so you assume ignition is fine and look elsewhere. 


I say start with the plug and work backwards to coil, cdi, etc.

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