1999 yz 125 weak spark won't start - 2003 Yamaha YZ 125

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They can do this from time to time. What can happen is the expansion chamber gets pumped with fuel vapor and then can be ignited, causing a large backfire. I hate to attempt to answer your question with a question, but how old is the piston and ring? A ring won't last very long in a 125. For racing, it's changed every race. For joy riding 15-20 hours of run time and it's time for a ring at least. Weak compression can make it impossible to start. If you have perfect compression the cause for the non-start could be something as simple as a burned up or fouled spark plug.

Start by re-ringing the motor (you can do this for about $25 inside of 2 hours time). Clean the carb out, clean the air filter, fill the tank with fresh fuel and replace the spark plug (NGK B9ES - from Autozone for $1.89). If the bottom end of the engine is good, the above steps should get it fired up - providing your ignition system is good, and based on the apparent backfire, I would say you have spark.

Scott

if your getting spark than i would take the carb off and clean it you could have a cloged jet.( just because your getting fuel from tank to carb dont mean its getting from carb to jug) i would try that first......yz-maniac.

if its a good spark it should start.try bump starting it,if no joy then possibly piston rings or other simalar.

If there was a probem with the coil the extra loading from the short circuit may have caused a problem with the trigger coil for the main coil. I don not have the spec but this value can be check with an ohm meter. Let me know how you make out...

it is one of a few things causing it to flail in the lower end, 1: your fuel oil mixture isnt lean enough.. 2: you need a higher spaking plug.. 3: your carburetor float level is out or 4: there is a slight break in the ignition spak line which would cause it to spark very weak until maximum accelleration is reached

Black spark plug means too rich , you will have to replace the spark plug since it has foul up and reset the air mixture screw on the carburator 2 and 1/2 turns outwards.
Check the air filter too.
Hope this helps.