Why does my car backfire through the carburettor?

Updated April 17, 2017

When your car backfires, it is causing damage to your engine as well as being noisy and embarrassing. Backfiring occurs when excess fuel explodes in the carburettor and is much more likely to occur in older cars than newer ones. If you have an older car there are some common reasons for backfiring that you should be able to troubleshoot yourself.

Lean fuel

When the engine is getting too much air or too little fuel, the fuel mixture is referred to as too lean. A lean backfire sounds like a rifle shot and can be accompanied by a small fast flame jet coming up from the carburettor. Lean conditions can be due to excess air from a vacuum leak, valve malfunction, or a leak in the intake manifold or gasket. They can also result from low fuel pressure.

Ignition timing

Each spark plug fires in turn in the intake manifold; the timing of this sequence is the ignition timing. If the timing of the ignition system is wrong, cross-firing can occur. When a plug sparks out of turn, it lights the fuel in the cylinder and the pressure will need to be released. If the intake valve is open it will backfire up through the intake manifold and out of the carburettor.

Accelerator pump

Backfiring through the carburettor can also be caused by a failure of the accelerator pump. If the backfire occurs as soon as you rev the throttle, the accelerator pump is the likely culprit. It is either not functioning, plugged up or out of adjustment. To check the accelerator pump, turn the engine off, look down through the carburettor and open the throttle quickly. You should be able to see fuel shooting from the sides.


Regular maintenance will normally prevent backfires. Your vehicle manual will contain a maintenance schedule. Change the fuel and air filters regularly, as a bad fuel filter can cause low fuel pressure and set up conditions for backfiring. Changing the filters regularly will also improve the overall performance of your car. Make sure your fuel injection system is tuned up and serviced on a regular basis. Check that belts are tight as loose belts can affect engine timing.

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