If it is blocked or partially blocked.
If the valve lash has closed up then fuel burns inside the muffler which will cause the muffle to glow red very quickly.
The valve gets more sticky and eventually the engine stops.
A partially sheared flywheel key can cause the engine to be retarded a little so the charge is still burning when the exhaust valve opens, however keys generally fail advanced.
A coil too far from the flywheel also retards the spark with the came result and generally it will be a bit more difficult to start.
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Engine model & code numbers ?