I don't have the techie answer for you, but I do have the proxy bike -- an '03 lowrider with 12,000 miles, screaming eagle pipes and high flow air cleaner. At fill-up today I was getting 44 mpg with 80% highway miles.

 

Wtf? Why am I only getting 23mpg? Granted most of my riding is less than 10 miles one way, and up and down hills in Pittsburgh, but 23mpg? Come on. My car gets better than that!

 

My 03 fxd got 45mpg with pipes and air filter,now with up grades I get 40mpg and I live in the Pocono Mountains and I run the s%#t out of it

 

How do you ride it? What RPM range do you cruise in?

 

Don't know the rpm range but I rarely get on it and wind it out. I never watched the rpms but it's gotta be pretty conservative. I only run about 60 on the highway in 5th gear. That's why this 23 mpg is baffling me. I purposely rode conservatively this last tankful and I actually got slightly less than 23 mpg.

 

Like I said earlier, if you are keeping the RPM's too low, your not using the motor efficiently and MPG will suffer. I would think you are keeping it low with a 5 speed, 60 is the low speed for 5th as I recall, I have a 31t trans pulley and I do not shift to 5th until 55.

Keep an eye on your RPM's and try to keep your cruising in the 2400 - 2800 range and see if that makes any difference. It does sound like you might be a tad rich with your jets as well, but not much, to be honest I have not played with a carb since 05 when I bought an EFI.

 

It's obvious that something is very wrong with your setup. There is no way you should be getting 23 MPG, even if you're running it outside the optimal RPM band.

I flog my bike pretty hard and average 29-31 MPG, depending on the ride, and my motor is pretty built.

If you don't have the expertise to adjust the carb, bring the bike into a reputable indy (or HD) and find out what's going on. I would think that you should be averaging around 40 MPG.

 

2 reasons come to mind... too rich of jetting and the reason your jetted so rich is because of an intake leak. No way you should need more than a 45 or 46 low jet. Probably a 45.

 

Something is wrong for sure, take it to a good indy. Long story short I brought mine to an indy because I was getting low to mid 30s on my bone stock 03 FXDL just like you are. The guy that jetted my carb said he has litterly done thousands of them and the instructions for jetting are usually wrong. 2 weeks ago I went from Missouri to the Rockies and back, MPG ranged from upper 30s to mid 40s depending on the wind.

 

Have you pulled your plugs to read them? There are charts online to help with visual inspections.