Valve clearceance still within limts? Honda ohc (CB as the chinese calls it) recommends in/ex 0.1mm for similar bore & stroke. Are Air filter plugged? Nothing cracked on intake manifold? Tried another new spark plug? The mixture seal lost easily if using air gun cleaning, the trick to quick fix (moile user need to cheat as PC to view the page ): https://www.motorfans.com.cn/forum.p...authorid=75082
Kehin clone PZ26 or Mikuni VM22 are plug & play for such 125...