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a fix point.
Correct, and it’s normally quoted in degrees before (or after) TDC.
make adjustments so the timing is correct as to load
and RPM.
That’s an oversimplification. What happens, is that timing changes as the speed control is moved up & down the range. The spark advance increases as the control is moved up through the range.
timing is controlled by you.
Ok, sure – we understand one another..
specifications because the mag plate isn’t fixed and is
constantly moving.
That’s a mis-statement. There most certainly +is+ a timing spec – and again, it’s normally quoted in degrees BTDC. On any motor, there will be timing specs for idle and/or full speed operation. Even in the case of manually variable timing as on this old Johnson, there are still lower and upper limits for ignition timing.
matched to the carburetor.
Ideally, timing is linked to RPM and load, which is why some engines have features like centrifugal and vacuum auto-advance mechanisms – but you’re correct that on this Johnson, the timing is directly tied to the position of the main carb butterfly. That’s a reasonable compromise, as the engineers figure that at a given throttle position, the engine speed will be X and as such, the timing should be Y. This is very reasonable on an OBM as the load is very predictable – all water acts about the same, driven by the same prop at a particular RPM.
The advance will at first be too high when the throttle is first moved from idle to full, but everything falls into place once the engine RPM catches up, which is (or ought to be!) shortly after the adjustment.
Having said all of this, there are still two hard datum to be known – the correct advance at low / zero throttle, and the same at full throttle. If the factory doesn’t quote it, that’s too bad – but it still can be deduced and measured using the correct timing tools.