Fuel economy?

How much fuel for 180km?  My guess is that you are pretty close to normal.

A few possible causes for poor mileage include a dirty air filter, leaking choke plunger seals, weak spark, dragging brakes, and using ethanol based fuel.

From the pic posted it appears to be a C3. A couple of things to keep in mind.  When the bike (1979) was new the fuel was different than it is now.  Info I have shows fuel capacity at 16.8 liters which equals 4.4645076849 US GAL

Nessism mentioned what I suspected (leaking choke plungers) as a source of increased fuel consumption.  Which is why I asked to check the plugs.  I wanted to know if they were burning correctly or if they. were sooty, which would indicate excess fuel burning.

The best way to figure out your fuel consumption is to ignore your tank size, and just compare how much fuel you actually use, and compare to your km each fill up. So fill up, reset your trip meter, ride it, and when you fill up next time, check your trip meter and see how much fuel you've used. It's all very good to look up the size of your tank, but we're not running it dry each time and we're not filling it to the brim each time either.

For comparison, my KZ750 LTD usually takes about 10 litres to fill from empty to full ('empty' being the low fuel light is on, 'full' meaning just below the bottom of the filler neck). That's for (what I believe) is actually a 14L tank. I'll get about 170km on a full tank, so around 6l/100km.

Thanks everyone for all the feedback and great info.  I did check the plugs (that were just installed new a few weeks ago) and they look good, not fouled or dark.  The air filter and fuel filter are also new as of 2 weeks ago.  I think I'll do a few test run fill-ups like you suggested going from full to empty and do some calculations that way to get a better understanding.  Maybe Im just overacting given my unfamiliarity with older bikes and that my newer bike gets such good mileage (350+km to a tank) and I'm simply comparing apples to oranges.

Thanks again,
 

You don’t have to wait till empty to fill. You just need to fill tank and visually document the fill amount. Rest the trip odometer, and drive, you can top of at any time just note how much fuel is needed to refill the tank to your documented level and note miles used from fill to top off. 

I recently switched to ethonal free gas for my car, (1997 Isuzu Rodeo V-6) after hearing about it in an earlier post here. I am getting at least 20% better mileage and the engine runs smoother, definitely.  It is more expensive and harder to find than regular but I'm sold. If you're in the states here's the site where to find it in your state.    www.pure-gas.org/

I bought my 1978 KZ650 new, and it got 60mpg every fill until the stupid first service at a dealer, which taught me a valuable lesson, as after that I was 40mpg. I should have never let the morons touch my new motorcycle. I don't feel like converting to KM, but in miles you should be good for 160 per tank even after dealer botch. 240 or so if all is perfect.

I own a totally stock 1977 KZ650-C1 that I bought new in early 1977.  I get about 40 - 45 miles per gallon if I ride it gently.  If I push it hard I get less than 40 MPG.  I've always gotten the same MPG; the bike now has more than 62,000 miles on it.  Ed