Q: Back firing

If you were working around the distributor and loosened and re-tightened it you might have set the timing wrong. Specifically, delayed or retarded timing causes backfiring as the spark occurs too late and so part of the air fuel charge is ignited as the exhaust valve is opening. Another cause of backfiring is the engine is being supplied more fuel than it needs to burn efficiently. With too much fuel to create an explosive, fast-burning flame, the fuel ends up burning slower, and the burn isn’t complete before the exhaust part of the engine cycle. When the exhaust valve opens on that cylinder, the extra air allows the unburned fuel to explosively burn and the popping noise of a backfire is heard. A defective distributor cap and defective spark plug wires can also cause backfiring but presumably the cap and wires were replaced during the tune up you did. A backfiring diagnostic performed by a certified Mechanic from YourMechanic would identify the root cause(s) of the backfiring problem your car is experiencing.