YZ125 diagnosis ideas

My '03 YZ125 has been running fine. Yesterday after riding for about an hour and stopping for about 10 min, it wouldn't start up. Double checked for fuel flow, ok, checked spark, ok; plug not loaded up, clean. Several hours later at home, bike started right up. I killed the engine. Would not start. Bike is stock except for an FMF Short silencer. Original top end. I'm doing a top end shortly, but need to find out what is causing this start/running problem before doing the top end.

Any suggestions on diagnosing the problem?

Bad kill switch. Always check the kill switch.

+1 on the kill switch. Also, take the pipe off and see if you can see if the piston may have seized, which would make it difficult to start, if it would start at all.

Good luck!:excuseme:

Thanks for the quick replies.

Kill switch is ok, sorry I forgot to mention that was the first thing checked. Piston hasn't siezed. However since it's due for a top end and I have the parts, will go ahead with that. At that time can check the reeds condition and for carbon build up on the power valve.

Anything else I should be looking for assuming the cylinder is ok?


bill t

change the plug! it might not seem bad but there could be something wrong with the internals that you cant see... might be starting after sitting for a while because your giving the fuel in the cylinder time to evaporate/ dry up!

im willing to bet thats your problem, just because its getting spark doesnt mean its good enough spark. for 3-4 dollars its worth the moeny to rule out rather than start to get frustrated!

Basic trouble shooting on a 2 stroke consists of fuel, fire and compression. Must have all three in suficient quantities to run. try to start and see if plug is firing, if so is it wet? if it's wet move on to compression, should be at least 100 PSi. I have seen plugs rhat appear to be brand new fail to fire when exposed to fuel and compresion. If you have all three and it still fails to start warm, it is time to examine the reeds, pipe and seals for leaks and blockages. I carry a spare plug when I go to the woods just in case I load up cause it's usually a long push home if I forget a spare. Good luck and hope this helps, cause a scoot with no poot is an agravation to say the least.

Thanks for the suggestions everyone. I'll give ya'll report once it's back running.

If any of the above mentioned stuff doesn't work I would look into troubleshooting the electrical system. You should be able to test the stator and coil, if you have a friend with a similar 125 you could swap CDI's.

Went ahead with the top end job. No joy there, still wouldn't fire. Installed different coil and CDI from Ebay....Wallahhh... Started first kick. Had a hard time holding back to break in the new piston/ring.

Thanks for all of your suggestions.


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