Check your air filter, they tend to be greedy if the filters are dirty. We once had a k11 that gave us 22mpg, and never registered above 1/4 on the temp gauge, turned out that it had the wrong thermostat fitted which was just letting full flow through all the time. Changed that, it rose to 38-40mpg.
Not sure if this is the problem with yours but if you have had a new thermostat fitted recently, might be worth checking.
Utherwise you might just have over-retarded ignition timing (had that problem too, setting it right made a big difference).