um CHILL 125 cc. Service Manual page 45

 Backfire
ITEM OPERATION
1 Pull out the spark plug, check whether the spark over is normal.
2 Check whether the timing is right
3 Check whether there is leakage of the valve
4 Check whether the injector is blocked.
5 Check whether the oxygen sensor works well
CHILL 125 cc. SERVICE MANUAL AND EFI DIAGNOSTIC
Yes
RESULT
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
maintenance
NEXT STEP
Next
Check the ignition system
Next
Adjust the timing
Adjust the valve
Next
Clean or change the injector
Next
Help
Change the sensor
45