199 YZ Piston replacement?

[lowriderhd]

My son has a 1999 YZ 125 and it started acting up. It started fine when it is cold but once it's been ridden even just for a few minutes it will not start up again unless you push start it. We removed the carb and cleaned it completely set it to the factory settings which was where it was set before we cleaned it. We cleaned and oiled the air filter. Removed the silencer, replaced the gas, replaced the spark plug, changed the transmission oil and still nothing. The carb has the stock jet. When we removed the air filter it would start a few times. Then that didn't work any more. When we first started pushing it it started and ran fine but each time it took longer to start and was bogging more each time. Now it won't start even with pushing it. I suspect it needs a new piston and rings as it appears not to be getting fuel drawn into the cylinder. The plug looks good and not fouled out. It is not wet either. The carb is set to factory settings. Does it sound like the piston needs replacing to anyone? Does anyone know what the compression should be on this bike?

Thanks

 

[flat capper]

a compression check is a quick and easy guide to wether it needs a new top end

 

[bobby bo]

agree, compression check. if the compression is good check the stator, make sure your getting a good spark. you can ohm out the stator with a multimeter...i dont know the values, but if you have a service manual it will tell you..

 

[kx125412]

Im almost positive you did this, but did you check to see if the gas was on? It wouldnt be the first time someone did this.

 

[Butcher-Boy]

Compression on my 99 yz with a new piston and ring is anywhere from 150-200 PSI depending on what compression gauge I'm using.

 

[lowriderhd]

Yes the gas was on.

Thanks

 

[sparkysakitas]

could be bad reeds also
also if it wasnt getting spark wouldnt the plug be wet?

 

[sick 96 250]

could be the piston/rings but you said the carb was at factory specs? Almost all 2 stroke bikes when at factory jetting are pretty rich. When cold and you start the bike sometimes it will start really easily because of the extra gas but when warmed up you could be flooding it. You said the plug doesnt look bad so that kinda throws off that idea but i would still mess with the jetting some. if not a top end it pretty simple to do