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look at plugs to see which cyls are not firing. put a thumb over the hole and run the starter see if you have compression on dead cyl. if you do take some sand paper and clean up plug posts and bands. simple stuff first. black means your taking in a lot of gas that your not burning. you shouldn't have to change settings you haven't changed. you can look at the valves and run the starter to see if they move a stationary valve should be easy to spot.