Injectors, timing, low cylinder pressure.
the valve guides could be worn out The carburetor is running fuel rich and needs adjustment. If it were valve guides or rings the smoke would be blue
Excessive revving of an engine causes overpressure in the crankcase. It causes unhealthy expansion in the pistons and the valves.
Unburned fuel in the exhaust system causes the backfire from the exhaust pipe.
Water vapour due to condensation in the exhaust pipe.
It is timed incorrectly or plug wires are installed incorrectly.
bad or going bad alternator belt
No who, but what. All internal combustion engines emit exhaust fumes.
Revving up and down is usually a symptom of a dirty air filter.
burning oil
Burning oil
Broke rings on pistons
gas from the exhaust