This should get you pretty close: Remove one spark plug. With some measuring device, set the motor past top dead center, with the pistons about 5/8" down from TDC. If you don’t have a tool for this, use a stick marked for TDC and 5/8" past that. Loosen the rotary valve until you can turn the gear, and remove the little plug in the top of the valve. Turn the valve until you can see the "J" in the plug hole. (This is a lot easier with the carb removed, so you can hold it in place while you re-install it on the motor.) Install the valve without moving the crank, with the "J" showing in the plug hole. Button everything else back up.
You may want to experiment one or two gear teeth in either direction, once you get the motor running. IIRC, advancing the valve will make it easier to start, retarding it will give you more top end, as long as the motor isn’t loaded up. The 5/8" value is for the S-45 and V-45, and I think the P series motors are different, but changing one tooth in either direction will probably make up the difference in measurement, and shouldn’t make a huge difference in performance. I hope someone will chime in with the exact measurement for the P-series rotary valve.
Videos, man, we want videos…
Good luck.
Tom
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