Before doing any work to the bike, start from first principles and determine the actual fuel consumption ie fill the tank, then the next time you fill up record miles travelled and the amount of fuel needed to refill. Then it's a simple calculation of miles divided by fuel (litres) equals miles per litre and then multiply that figure by 4.54 to arrive at miles per gallon. Once you know the mpg, then you can decide if it is lower than you expect and if you need to investigate doing any work to the bike.
