My airbox had about a half a cup in the forward part of the airbox (forward of the 2 holes in front of the filter element). Investigating a strong smell of gas, I took things apart until I saw the gas. Cut a small hole at the base of the wall below the 2 holes near the bottom so any gas buildup in the future would go down the airbox drain. In my case, I think a float was sticking at some point (it was running very rich and would die at a stop) and managed to have gas flowback into the box. At least that's the only way I could figure it got there. After the running problem resolved itself, there were no more gas in airbox probs.
The reason you have gas in the airbox may be different but whatever the reason, I don't think it's normal.
