Depending on the power level you are running the engine at, you should expect a simple 2 stroke such the Victa ones to use about twice as much fuel as a 4 stroke of the same rated power output, when mowing lawns. It varies from only using about 25% more fuel at maximum power, to using about 4 times as much fuel at idle. Here is a comparison of 2 and 4 stroke outboard engines (and note that those Suzuki 2 stroke outboards are far more fuel efficient than Victa mower engines):
http://www.outboard-motors-and-boating-geraldton.com/support-files/fuel-efficiency.pdf
