Since I put the proper sized rear tire on the GS I decided to ride it for the week before putting her on it to check for any other problems it might have. The bike was great to ride for the week, lots of fun back and forth to work. We go out for a ride over the weekend, she puts a few miles on it, takes a turn, and it stalls out. We pull off, she restarts it, makes it a little further and it stalls again. I flip it to the res. and it runs fine. I only put 90 miles on the bike during the week, and we'd only put 25 miles on it before that since the last fill up (to the top of the filler neck). So less than 125 miles before I'm having to flip it to the reserve. We rode to the gas station and put 3.4 gallons ($10 worth) in it which brought it back to the top of the neck. I know something HAS to be wrong about this, I though the tank was almost 5 gallons and the bike was supposed to get around 60mpg. Hell, my 650R gets 150miles before the low fuel light comes on and 58mpg. The bike doesn't "seem" to run rich, I don't get any smoke from the exhaust and its not sooty. The brass plug still seems to be in the carb so I don't think the mixture has been played with. Idle is around 1300.
What things could cause me to get this kind of milage, the bike only has 6000 miles on it, air filter looks like new, I run 93 octane fuel.
check your exhaust valve clearance. If that is off, you might have fuel getting into your pipe and detonating. I get around that same mileage and I have that problem. On a side note, your MPG goes down if you're running around wide open throttle, or cruising at high RPM.
Also, open up your float bowls and make sure your jets are screwed in. My bike was running great one day. So great that I got worried. I opened up the float and one of the jets was resting in the bottom of the bowl.
For the valve check, you can either drop around $200 at a shop, or you can do it yourself. This video was really helpful.
http://video.google.com/videoplay?docid=-2015554469142545363 (http://video.google.com/videoplay?docid=-2015554469142545363)
Also, I was wondering why you were using 93 octane. As far as I know the bike was designed to run on 87, so I don't know if high octane helps.
Was this a one-off or does it do this consistently?
If it's only happened once then you may have been the victim of the crapped-out petcock problem that afflicts the GS (lots of links in this forum). It's happened a couple of times to me, but not since I got into a little routine on start up: turn to PRI when starting and warming up, just before I ride away I turn it to RES and then all the way back to ON. I get 150-180 miles out of a tank, depending on how I ride... ;-)