Why is my Johnsson 9.9hp, 2 stroke using so much fuel?

I put in 5ltrs (1Gal) the other day. In all I think it ran for about an hour and a quarter..is this the best I can expect.

Plus.on the front panel, to the left of the pull cord, is the choke and another knob..what is the fuction of this knob?

Answers:
does the exhaust smell strongly of petrol ? could be that you've got the mixture too rich ( too much fuel going through )

the knob could be to alter the mixture have you tried running the engine with it fully extended / retracted ?

if in doubt take it down to your local garage
You ran it for an hour, but at what throttle setting - idle or fast? What kind of boat are you pushing? How much load?
Does the "other" knob turn? That's to adjust the mixture for hot or cold running. You'll have to verify the motor is tuned up correctly to get optimum gas savings.
1 1/4 hour full open? I think you did well.
Remove extra weight from the boat.
The engine uses the most gas at wide open throttle and might be more efficient in terms of Miles Per Gallon when it is running at 2000-2500rpm

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