Normal and typical engine bubbling after installing an accessory exhaust system

In a large number of cases, the typical engine bubbling that becomes more noticeable after the installation of accessory exhaust systems is mistaken for misfire. The engine bubbling is normal and occurs more or less strongly with all petrol engines. In old carburetor engines, engine bubbling occurs more often because the engine draws fuel through the idling system when the throttle valve is closed.

Engine bubbling occurs because the amount of air sucked in when the throttle cap closes suddenly is not sufficient to burn the metered fuel. This burns relatively slowly and ignites audibly after opening the exhaust valve in the exhaust duct and exhaust manifold. This mechanism is prevented by the overrun fuel cutoff, but the remaining amount in the combustion chamber may be sufficient for a short re-ignition. If, for example, a gas surge is briefly given during push operation, the overrun cutoff stops for this moment and the process described begins.


Misfires are not uncommon or unusual and can have a variety of causes that are independent of the exhaust system. Therefore, they do not necessarily indicate a deficiency or a defect.

Common causes of misfires:

  • Volume / pressure changes: If the exhaust system has been installed for a year or more, there is a possibility that the volume and thus also the back pressure have changed due to the combustion of the insulating wool. A new insulation of the exhaust system and the use of a quiet dB absorber (available from us) could alleviate the problem, since then there is an increased backflow. You can get high-quality insulation wool including gas-tight rivets from specialist retailers. We recommend having the muffler replugged by a professional workshop near you. Please note that you cannot send your exhaust system to us for new insulation.

  • Leaky exhaust system: Is your exhaust system possibly leaking? To do this, check the manifold and cylinder connections.

  • Old spark plugs: Old spark plugs can also cause misfires.

The causes of misfires are diverse. If you are unable to identify the problem straight away, you should have your vehicle checked by a professional workshop.