Exhaust restricted or plugged
If the exhaust is plugged or otherwise restricted, the exhaust gases will not be expelled completely from the combustion chamber. If the combustion chamber contains exhaust gases during the intake stroke, fuel may build up in the intake manifold and alter the ratio of fuel and air. This will reduce engine performance and can lead to stalling and backfiring.
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A restricted exhaust cannot clear the exhaust gases from the combustion chamber. This will result in an alteration of the fuel/air mixture that reduces engine power. The diminished power may make the engine struggle to maintain the momentum of the crankshaft.
Unburnt fuel may also exit into the exhaust manifold where it can be ignited later. This will result in a backfire.
The reduction in fuel ignited in the combustion chamber will also reduce the engine power. This can also lead to stalling.