try using b-12 chem tool in the carb to get it started and just try a new plug and u seid it might have a bad piston if so just get a new ring to go around it that will give it more compression if that chem tool dosent get it started than its not the carb im not sure about the 125 but i have an atc 200 and 110 1982 but also if it has a timing chain try going up a toothe or down one that might be why its not firering right.you dont have to do a complete rebuild on the carb that just will cost u money same thing with oil but make sure there is oil tho.o yea when you spray in somthing in the carb to get it to crank hold the throttle.and try your wiring is good somthing might be shorting out if so that would be the problem expesially the wire by your points


ATC THE GREATEST LEGEND
THEY MOST OF DID SOMITHING RIGHT IF THERE STILL AROUND
IN THE 20TH CENTURY IF THEY LASTED 25 YEARS AND STILL COUNTING

 

im going throught the same thing,even put one of those new china carbs on and it ran for a few then it doesnt want to run.if the battery is dead,will that cause an issue?

 

Sounds like your timing chain jumped a tooth. Pull cover off of flywheel and remove ignition cover and rotor so that you can see the cam gear and chain, and make sure your timing marks are lined up, then check your tinming chain tensioner (on bottom of engine). Zero backlash on the chain! you need to get a Clymer's manual so you have step by step on how to do this stuff. Helped me thru my learning curve tremendously!