The first year that I rode my 2004 GS 500F I could get almost 300 miles on a tank of gas.  Within the last year I have only gotten about 115 miles on the odometer and then my bike will only run with the reserve tank.  I have had several other problems fixed to include: cleaning clogged carbs, having them rebalanced, and also having a jet kit installed, but this issue of terrible gas mileage has remained.

I have been reading that it could be my float bowls are set too high.  If this is the case, how involved is it to re-set them?  If there are some other reasons this might be happening, any thoughts or suggestions would be greatly appreciated.

Thanks,

LBB

i've never got anywhere near 300miles  but i've lost about 20 miles on my tank in the last weeks. 

Hmmmm.  Float bowls too high = fluid pressure is too great and pushes gas through the jets easier.  Possibly.

Do GS have an enrichment circuit for the carbs?  If so, check those parts- My Shadow was running crappy, it was a leak in the enrichment needle, the seat becomes hard and no longer allows the needle to shut.  Essentially, the "choke" was on all of the time.  Pulling spark plugs showed one cylinder was RICH.  I think the principle is the same, different bikes, both have CV carbs.

Have you checked the plugs?  Fouled and sooty plugs = too much gas. 
I wonder if carbs need cleaned- disassembled and all passages cleaned.
How many miles on bike? 
How does it run otherwise- how does it feel?  Does it take gas, does it run at high speeds, does it accelerate...

I think that adjusting the float level is not that big of a deal.  It's just knowing what the level is supposed to be.  Check this site, this sounds like a problem with a simple cure.

Leaking float valves?  They are small, delicate parts, the rubber tip becomes hard and won't shut off gas- maybe they are leaking?  Allowing too much gas to pass by?

Well, I'm all out.  Good luck.   

Carb parts are exposed to gasoline, and also many heat and cooling cycles.  They age just like any other part.

After rejetting your gas milage will go down a bit but not that extreme.  What jets are you running now?  Did you do any mod to your air box or exhaust?

Max: That's 300 including reserve.  I've gotten 320 to completely empty before rejetting.  I haven't done any calculations after rejetting because I'm having too much fun to worry about fuel economy.

oh i see  . how many gallons can you actually get to once you switch it to reserve?

On the 01+, tanks have 5.3 gallons total (49 state).

4.3 pre-reserve, and one gallon of reserve.

http://cgi.stanford.edu/~sanjayd/gs500/Main/Fuel

You have to drive like an old lady to get 60mpg.  Lately I've been getting 50-55, so I haven't seen 300 on the odo much anymore.

most people can only put in 3.5 instead of 4.3 gallons, but that shouldn't mess up the 1 gall reserve right.

Jlabax, I don't think you mentioned this:  You say now you have to switch to reserve at ~115 miles.  When you fill up, do you are you still putting 4-5 gallons in the tank?

The last time I filled the tank after having to get home in reserve it took about 3.75 gallons and that made it full.  I definitely don't drive like an old lady, but I'm not getting anywhere near 50 mpg.  After I've switched to reserve I have never emptied the tank fully to see how many miles I get, but at one time when I hat to switch over, I was 15 miles away from home and I made it just fine. 

As far as maintenance is concerned, I had the carbs cleaned and rebalanced about a year ago and it didn't solve this problem.  The K&N Jet Kit I had installed a few months ago, but this gas mileage issue existed way before having that done.  I did not have any mods to my air box or filter, but they did replace the air box.  Spark plugs were checked about a year ago and the oil was changed probably 8 months ago or so.  The bike is a 2004, but only has about 2300 miles on it.

The bike runs great otherwise.  I have no problems with acceleration or running at 80 mph on the highway.  It shifts fine, idles fine.  It just seems to cut out like it's out of fuel at around 115 miles.  As long as I switch to reserve, I can get going again.

Thanks everyone for your input.

that's about where i have to switch to reserve, running 40/142.5 jets.  if you're still concerned --- brakes dragging?

I have only had to switch to reserve on the first tank on mine and I don't think they topped it off.
That was at 195.....I have been averaging at least 55mpg but I am basically all commute highway miles.