I've seen a few people having issues with putting the air box on the carburetor. I did too. I also noticed the big end was soft and pliable while the other end was hard as a rock. I heated it up in hot water and stretched it over a piece of PVC pipe which made it a few millimetres larger. Then did it again, stretching it over a large socket. Once it cooled, it held its shape and slid right on the carb.