Symptom Diagnosis: Backfire

Check thermostatic air cleaner operation.
Check accelerator pump operation.
Check carburetor choke, vacuum break(s), linkage and unloader operation.
Check air injection system control valves.
Check for crossed plug wires. Check for cross-firing inside distributor cap due to carbon tracking.
Check for intermittent failure of pick-up coil or ignition primary connections.
Check EGR system.
Ensure compression is correct. Check for sticking or leaky valves.
Ensure exhaust system is not restricted.
Ensure valve timing is correct.