I would think that problem would be more likely caused by something sticking, such as the throttle linkage or the armature plate (timer base) under the flywheel... slowly returning to normal.
Best way to check for sticking is to run with the hood off and have someone staring at those two areas as another member closes the throttle.
The full advance spark setting is done via a timing light and the numbers on the flywheel... whatever setting it states in your service manual... and can be done via the method stated below. Your engine DOES NOT have the "Fast Start" or "SLOW" feature so don't be concerned about that. If you or someone has been tinkering with the timing.... yeah, check it! We wouldn't want you melting a piston!
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(Timing At Cranking Speed 4°)
(J. Reeves)
NOTE: If your engine has the "Fast Start" feature", you must disconnect/eliminate that feature in order to use the following method. The "Fast Start" automatically advances the spark electronically when the engine first starts, dropping it to normal when the engine reaches a certain temperature.
The full spark advance can be adjusted at cranking speed,"without" have the engine running as follows.
To set the timing on that engine, have the s/plugs out, and have the throttle at full, set that timer base under the flywheel tight against the rubber stop on the end of the full spark timer advance stop screw (wire it against that stop if necessary).
Rig up a spark tester on the #1 cylinder plug wire. Hook up the timing light to the #1 plug wire. Crank the engine over and set the spark advance to 4° less than what the engine calls for.
It's a good idea to ground the other plug wires to avoid sparks that could ignite fuel that may shoot out of the plug holes. I've personally never grounded them out and have never encountered a problem but it could happen.
I don't know the full spark advance setting your engine calls for, but to pick a figure, say your engine calls for 28°, set the timing at 24°. The reasoning for the 4° difference is that when the engine is actually running, due to the nature of the solid state ignition components, the engine gains the extra 4°.
If you set the engine to its true setting at cranking speed, when running it will advance beyond its limit by 4° which will set up pre-ignition causing guaranteed piston damage! You don't want that to take place.
No need to be concerned about the idle timing as that will take care of itself. The main concern is the full advance setting.
A fellow member from one of the various marine forums suggested having water supplied to the water pump (flushette or barrel) simply to provide lubrication to impeller. A worthwhile suggestion I thought, and entered here.
Be sure to use your own engines spark advance settings, not the one I picked out of the air here in my notes.
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