I have a 91 model GS500 that I purchased recently. Bike is running good overall. However, I am getting very low fuel mileage from the bike. From what I have red over the internet and heard from some friends GS500 gives at least 25km per litre of fuel average while my bike has given me hardly 13km per litre, almost half of what it should. Q 1. Do I need to check the carbs?
Q 2. If I get carbs of later model GS like 2009 model or so, will it be compatible with my bike (91 model)?
Q 3. How much new carbs should cost?

Thanks..

Start by checking the spark plugs. If they look good, it's doubtful that your carbs account for your poor fuel economy.

If that isn't the issue, put the bike on the center stand and ensure that your brakes aren't dragging, and that your bearings are good. It's best to do this right after a ride, in case you have boiling induced brake drag. Also inspect the condition of your chain.

And if you are unsure, post a picture with a description for us! many people here can tell you the problem from a good pic! (plus we're all pic whores on here  )

Sure. I'll check the plugs and brake pads and share the results.