Spray some starting fluid in the intake with the filter off and choke off and see if it fires for a few seconds on that. If it does its a fuel flow issue. Petcock could have gone bad when you turned it off to remove the gas tank. Check all the easy stuff. Kill switch and all that stuff. Cant tell you how many times i have missed something that just ended up being simple. Did the gas tank get set in a way that fuel could have ended up in the tank vent and be not allowing fuel to flow into the carb? Try removing the lid and see if it starts then. You only remove a few things to get to the adjustment covers so our options are limited. Try tapping the side of the carb. Maybe the carb float got stuck closed and isnt allowing any fuel to come in?