The engine problem can be static, uniform and non-rhythmic such as constant engine shiver and shoulder, or the engine vacillates between smooth and rough times with noticeable rhythm. Differentiating the pattern helps a lot in diagnosing the problem.

 

If it has static and uniform pattern, you may check the ductwork for possible cracks and the air filter for cleanliness, which can trap a fair amount of leaves. Also check the exhaust system thoroughly. If the engine seesaw back and forth rapidly between calm and rough running is rhythmic and the problem could be the cylinders.  

 

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