I rode to a rally last week with a friend who has a 2001 (or 02) Fat Boy - 88 ci V-Twin with carb

The bike just started getting reall bad gas mileage over the past month or so - like less than 100 miles per tank at 80 mpg on pretty level road.  Prior (a couple months back) she was getting 120-140 miles per tank.

Nothing was done to the bike recently.  She did put new pipes and had the carb re-jetted a while back, but after that she was still getting good mileage.  We checked for the obvious things we could on the road like gas leaking, clogged air filter, etc.

Any idea what could cause the gas mileage to drop like that?

I'm thinking there's a bad fuel or crank module goin' out.

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.....

.....ooh, just thought of something else too...depending where carbs were made...it could be a worn out carburettor...some are made from the cheap and nasty aluminium from korea.....contains a higher percentage of magnesium...gasolene can erode this content on the surface walls inside carb...check for any signs of wear through carb too ok?

If it looks pitted or salty anywhere in carb its one made from the cheap shitty -oops, I mean nasty stuff from korea.....my jet engine dont just melt that stuff...lol...it BURNS IT UP to a fizzing mess!!!  Ya seen what I do to twist and go scooters right?

Oh I'm sorry....I guess I didn't see the word "carb"

Any change in the type of gas being used in the bike?