IMO I would start by draining all the fuel mix out of the tank completely, take the tank off and make sure no debris is in there also, pull both plugs and take a compression check, take the carbs off and pull bowls off them checking the floats and needles and seats, and jetting. Then pull the exhaust from the heads check for carbon build up and fix any exhaust leaks you have all the way back, and get your silencer repacked.