It's dropped a fair bit but still need to get another 600+ ppm down from the NOx reading. What are your plugs like? Have you changed fuel (gone up to a higher octane fuel like 98+)? You can try decarbing the engine by taking the air filter lid off it and spraying water with a trigger pump down the throat of the carb while running it with the throttle open. This won't kill it and you should be able to hear some exhaust 'crackle' when the carbon build up is burning off.