You need to get the carb seated properly. As the other poster already said.

Take the prop off. Loosen the bolt and nut that holds the carb on. When you loosen the bolt and nut, make sure and wiggle the carb back and forth. It sticks into the case when you initially tighten the bolt. You need to make sure it's nice and loose before you squeeze or smack it down. Take it out if you have too--and put a little 30w motor oil or sewing machine oil on the carb shank first. Any type of light lubricant will work. This will ensure that when you put the carb back in--it'll slip and slide easily without binding up. Now grab the front of the engine--with your thumb on top of the carb. Squeeze the carb down onto the case as hard as you can squeeze it. Hold it down and tighten the bolt and nut. If the O-ring is flat, then you go it sealed up. If it still looks round, then you aren't squeezing hard enough.

FlyboyDave does something similar, but he uses a small hammer and a block of wood. I think he just sets a block of wood on top of the carb and then gives it a few light taps with a hammer to seat the carb down tight. Tighten the bolt and nut.

That should fix your air leak. If it doesn't work, then use the above method of cleaning with Gumout and then use red RTV silicone on the carb shank before setting it into the case.