Robborider,
If fuel consumption is that poor, perhaps it would be wise to get the injector checked out. It may be just that one aircraft.
If it is the same injector as an Enstrom (Bendix RSA) then correct adjustment can make a huge difference to fuel consumption but no apparent difference to performance - until the plugs foul up.
I have flown Enstroms with big differences in the full rich consumption.
Incorrect injector adustment is less of a problem on an Enstrom as hitting the lean knob in flight is routine. I don't know if anyone dares to do it in a Robbie.