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Exhaust pop nature

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Hello guys! I want to discuss with you the nature of exhaust backfiring. Usually it's either too lean or to retarded ignition. Never seen pops becuase of too rich mixture,have you? And usually (especially on bikes) exhaust air injection system helps this pops. It adds air from filter box after axhaust valve. Why it pops? Is it because of leaning mixture in cylinder or fuel burns in exhaust pipe? Is it connected to burning rate of lean AFR that burns slower?

hello it pops because of the added oxygen into the exhaust you will also get it from an exhaust leak or retarded ignition timing like anti-lag where the exhaust is open already and the ignition is triggered to fire the sparkplug

Thank you, Ross! I understand that's because of exhaust leak but how leak works? Is it like afr enleanment or not?

Backfiring, and that nice rumble/cackle, you sometimes get on the over-run is due to the fuel-air mixture not burning properly in the combustion chamber, but either continuing the burn in the exhaust, or not actually being ignited in the cylinder but in the exhaust by other hot exhaust gases.

Air drawn into the exhaust can also contribute if there is unburned fuel as the oxygen allows that to be ignited - it is far from a requirement, though.

Possible causes are intake leaks causing a lean condition under high vacuum, overfuelling causing an over-rich condition, the spark plug not igniting the mixture (may be worse with smaller gaps) and/or another factor.

As a general rule, which may help diagnosis, is that a lean misfire/backfire will have a much sharper sound - a crack like a rifle - while a rich misfire will have a 'softer' tone, more of a 'pop'. If it sounds more like a shotgun, really loud bang, it may be ignition based with the exhaust being filled with a fuel-air mix.

That last can be a problem with carbies as exhaust explosions after shutting the engine down, especially with rotaries, occur because the engine was still drawing in fuel-air and passing it into the exhaust as it slowed down, and any spot in the exhaust hot enough will ignite it.

Thank you for detailed answer, Gord! I am talking about motorbike engines, but I think it doesn't matter. I tryed to make exhausted backfire with overly rich mixture and it was almost no pops usually. With enleanment it usually works.