yeah, 500's are prone to dropping valves if you flog 'em too hard but people still like them if they can find them intact.
Putting carbs back on my 86 bikes is an exercise in balance and futility. Usually I tie the handlebars to a roof rafter and stand on top of the carb plenum with hands on the same roof rafter for balance. Then I pretend I am on one of those balance discs at the gym and my fat ass seats it or it pops out and I have to go through the miserable process again and again. Yes, it sucks. I do not do it that often and am glad I do not have to.