B&S won't start - mower

craftman 5.5 HP with diaphragm carburator.
Changed: Oil, fuel, spark plug, coil, diaphragm, carburator gasket, air filter
Verified: Keyway, compression = 90psig, oil level, primer pump squirt, spark is fine enough to electrocute me... trottle linkage.
Cleaned: carburator, fuel tank.
3 years old
Symptoms: Won't start, sometimes puffs, sometimes backfires through carburator. Spins freelywhen attempting to start.


I am nowclueless... Do I need to open then engine?

Thanks for you answers...

Chlag