I just bought a 2003 yz 125, from a friend today. I knew it did not start but for $850 i wasnt going to turn it down. So, i took the carb apart and cleaned it. Still didnt start, so i put a new spark plug in. Still didnt start, so i did a compression test, and i found that i have more than enough compression. The motor has 2 hours on it, and the gas in it was 32:1.
Spark
Did u check if it gets any spark at all?
fixing the bike
check the piston and then see if the plug is getting any power
If you are getting any spark
If you are getting any spark within the ignition system, you may want to double check the carb setting. Try to spray some starter fluid and try to start it. Also set the a/f mixture correctly.