Before you do any of that check it is not a vacume leak. It could be air getting in after it has passsed thriugh the carbs, This is normally a leak on the rubber bits between the carb and the cylinder head. A quick way to check it with a quick squit of carb cleane3r over the places that air might get in. Just a quick spray will let you know, IF AFTER SPRAYING THE REVS CHANGE THEN THAT COULD WELL BE YOU PROBLEM. jUST DON'T LET ANY SURPLUS SPRAY SIT ON THE PAINT WORK AS IT MIGHT DISCOLOUR IT.. Sorry about the capitals but could not be bothered to chage them.