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I see this all the time.
The fuel /air mixture is off due to lack of combustion air.
You need to clean the burn pot and all the holes, take off the side and rear panels of the firebox and also clean.
The exhaust pipe needs to be disconnected from the exhaust manifold and cleaned.
This should do it. remember to adjust the side air damper at whatever heat output setting your running at so that the tops of the flame are burning clean with no carbon.
If the problem is still happening after all this it may be a combustion fan failure.