The previous owner probably did not change the jets or do any carb mods (most don't). I'd leave it and see if the missing subsides as its may be just a carbon build up from running lean w/ the muffler holes or perhaps a worn out spark plug. Wind it out for a few days and see if that makes it go away before you have it looked at. If you use gas without ethanol, you'll probably get into the 68 to 72 mpg range with conservative driving. It's about 6 +/- mpg lower w/ 10 percent ethanol.
