Jorge,

I don't know this engine, and have only limited experience of 4-cycle engines.

Because of the valve train, 4-cycle engines have practically a built-in RPM limit. Too fast and valves float. Not good in a car, not good in a model engine... So, they do not speed up if the mixture goes leaner. I have heard that a safer way to set the mixture on a 4-cycle is with a tachometer. As the mixture is leaned from very rich you may find RPM increases at full-throttle. If so, you will see the RPM stop increasing at a certain value, even though you can make it leaner. My opinion: the richest mixture that comes close to that RPM should be safer than a leaner setting...

I've noticed that some engine manufacturers recommend less oil for their 4-cycle engines than for 2-cycles. Also, quite often, synthetic oils or blends of synthetic and castor are recommended. 22% castor seems a lot. What does the manufacturer recommend?

How did the engine feel after it cooled down? Have you checked the valve clearances since the very hot run? Did the carburetor drum (or plate or butterfly) actually turn as the linkage was operated? (The linkage arm could possibly slip...)

Does the cowl have enough opening for air to enter, AND a bigger opening for hot air to leave the cowl? I hope the slow running was due to the carburetor arm slipping, and not to binding of overheated parts inside the engine.

I hope your engine is still in fine condition, and that you have long and pleasant service from it.