I have a 23 HP Kawasaki vertical engine FH680v that is not running right once the engine is run for 4 or 5 minutes it starts to mis fire and blows a small amount of black smoke each time it surges this engine has been well serviced for 4 years I have changed several parts . New carb, new plugs new coils, new air filter new fuel pump . Also the plugs are jet black after just a few hours of use when i installed new ones . The engine has 1500 hrs of use could this thing be ready for a new engine . All help would be great ! Thanks Im losing sleep over this one.