It looks like you are necking a 2" pipe down to a 1" pipe. In my experience when you neck down a blown burner so drastically you have a hard time getting a stable flame because it likes to blow off the end of your burner. Try shortening your air pipe, you only need it half as long as what you have. Cut your 90⁰ bends down to two, you don't need any more than two. If your necking down a 2" pipe only neck it down to 1 3/4" or 1 1/2" or leave it un necked down. It it's a 1 1/2" pipe only neck it down to 1 1/4" or leave it un necked down.