Hello all I need help please I have a 96 trail boss 250 that won't start. I've replaced the battery, new plug and wire, cleaned carb and replaced fuel lines and replaced cdi box. The engine cranks and cranks until it back fires through the exhaust. I just checked the stator and it looks and appears to be fine with a meter. The cdi box that I installed was a used one. I'm out of ideas. Please help

 

What was the compression reading? If lower than 120 psi could be the problem,plus crank seals leaking that people forget about.