Advance curves for outboards are difficult to find, for reasons that Tubs has stated. Here is one that I have found. It is from a paper on the state of the art in outboard motor technology (1963). The paper was published in England by the Institution of Mechanical Engineers. It shows ignition angle from about 6 degrees ATDC at idle, to around 37 degrees BTDC at full speed. I would speculate that this is a representative ignition curve for a broad variety of outboard motors.
