When I've had air leaks, it has been one of two places: the carb wasn't fully on the manifold and tight, or the top (the throttle slide cover) wasn't tightened down. You can tell if you can get it started, then spray carb cleaner or WD40 on the carb while it is running. If the engine idle changes, then there is a leak. It sucks the liquid in through the leak and then burns it, changing the fuel/air mix.
Good luck.
Chris