Do you have any ratcheting tie downs? If you do, use it to pull back the airbox. You might also be able to keep them pulled back with some rope.
Another way it to have someone pull back the airbox while you pull out the carbs. Wriggle them a bit when you pull out. Sometimes using the handle of a clawhammer (rubber not wood natch) like a fulcrum can help ease them out. If none of this works, heat up the boots with a hair dryer.
Oh yeah, you should also take the tightening bands off the airbox boots before you try to pull the carbs out. Very easy to mangle them trying to get the carbs out. (Ask me how I know.
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