I have older lawnmower that run like a charm till this past weekend. Now when I started it it did not run even, very erratic and black smoke coming out of the muffler so I figured the gas mixture must be too rich. After some time I figured that the flap in the air intake under the air filter is not opening up to allow more air to mix with gasoline. When I took off the air filter and used the screw to manually open that flap the engine picked up and run nicely, when I let it go it goes bad again. How does that flap normally open? Is there mechanical link to force it open or does the increased airflow suppose to push it open? What can I do to fix it? Here is a link to the video file I have done to demonstrate what is going on: http://www.mcmasteraccommodations.ca/LawnmowerIssue_1.mp4 Any help will be greatly appreciated.