Q: 1978 CJ5 304 Engine stars back firing after it warms up after a long drive on the highway it stars backfiring

If the supply of fuel to the carburetor is reduced once the engine is hot, you might have a vapor lock condition. You might evaluate the location of the fuel lines and relocate as needed. Backfiring is often caused by a too lean or too rich air-to-fuel ratio although a few other causes are discussed below. A backfire in the intake manifold is caused by a ratio that is too lean (not enough fuel). A backfire out the exhaust system is caused by a ratio that is too rich (too much fuel). If backfiring is occurring, it is recommended that you request a diagnostic to identify and resolve the root of the problem. Some other possible causes of backfiring are moisture or resistance in the car’s electronic ignition system that allows a crossover generation of current to the wrong cylinder; a lean mixture caused by either an engine vacuum leak or low fuel-pump pressure; a faulty air injection check valve (if equipped; part of the car’s emission control system) allows too much air into the car’s exhaust system; faulty ignition components or spark plugs may also cause backfires if the ignition timing drifts off or the plugs fire intermittently; and an exhaust leak such as broken welds or other small holes can take in cool air during deceleration and cause a popping or backfiring in the exhaust system. If you request a backfiring diagnostic the responding certified mechanic will get this taken care of for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.