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

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I have own a 1978 Jeep CJ5 since 1984, it has a stock 304 engine with around 82k miles. For a while now whenever I drive specially on a long drive on the highway it stars backfiring and it seems like is not getting enough fuel in the carburetor, after the engine cools off for a little the problem goes away. I have replaced the following: Carburetor Fuel pump Fuel filter Fuel sending unit Electronic Ignition Distributor cap and rotor Spark plugs and wires Muffler (removed catalytic) The timing is on, no vacuum problems The problem still persist, don't know what else to do.

My car has 82000 miles.
My car has a manual transmission.

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.

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