Bent exhaust pipe
A bent exhaust pipe will distort the flow of exhaust gases. The gases can build a back pressure and begin to flow back into the combustion chamber. The closer the bend is to the exhaust manifold, the more likely the gases will reenter the combustion chamber.
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With a bent exhaust pipe, the flow of exhaust gases from the combustion chamber can be restricted. This will build a backlog, and even allow the gases to return to the chamber. The fuel/air mixture will be mixed with the exhaust gases, leave some fuel unburnt. This fuel can then exit the exhaust valve and be ignited in the exhaust manifold. This will cause a backfire.
If the exhaust gases cannot fully escape, or are returned to, the combustion chamber, the engine will not be operating efficiently. The fuel and air will be mixed with the exhaust gases, reducing the power exerted by the fuel and air alone. With a significant reduction in power, the engine will not be able to maintain the momentum of the crankshaft and stall.