Next on the list of factors is pressure. When you increase the surrounding pressure, you can usually dissolve more gases in the liquid. Think about your soda can. It is able to keep the fizz inside, because the contents of the can are under higher pressure.
Assuming a prop, it depends on how you are propped but with a redline of 6000 rpm, most outboards perform best with a cruise in the 4000-4400 rpm range. If you want top fuel economy, get a floscan or such and see what is the most efficient for your engine and prop.
First cheap thing to do with an outboard motor is replace the spark plugs. They always foul out because of the oil/fuel mixture used in a 2 stroke motor. Don't start measuring voltages. The next thing is making sure you have fresh fuel. Ethanol in the gas is bad enough when it's fresh gas.