Take out air cleaner, wash it in gas, then wash it with a good brand of de-greasing detergent, use an airline to blow out any fine particles that will still be in it after washing, blow it from the inside outwards, then get a light oil and resoak it, drain it, dry excess oil off with rag, even shake off any excess oil before ragging it, fit it back on and go again and see what milage you get.....
If milage becomes good again - get new air cleaner or be prepared to repeat the process over and over like I do.....lol.....
Normal aspirated Harleys need a bit of TLC on the airflow side of things.....mine still has its ORIGINAL air cleaner, battery and belts!!!!!
Just a bit of regular maintenance on those things because if you are living in a very fine dust - dusty area it will need doing almost monthly.....
Belt drive gets done with a toothbrush regularly.....
Battery is under constant charging even on idle as my idle speed of 800rpm is slightly higher than the usual "Potato Potato Potato" idle speed of 600rpm...
Hope this helps.....