Engine knock comes from preignition, the fuel explodes before the end of the compression cycle. It is caused by high compression and a fuel that burns too fast. We get confused but high octane auto fuel eliminates knock by burning slower. And you may be getting some backfire to throw props. Get a lock nut from the hardware store for safety.

You can also easily get knock if mixture is too lean. To high a ratio of oxygen to fuel vapor causes fast ignition. I suspect this is part of your problem since problem goes away when you richen up.

Here are some thoughts.

Run it rich, especially if it is new and in break in. The knock may go away when you finish break in.

Check the valve setting.

Colder plug may slow down ignition.

Add some castor to fuel. Burns slower.

Use 5% nitro. Burns slower and should be all you need, especially for a young engine. Don't worry about nitro to sweeten up your idle. A new engine will idle a bit fast until it breaks in.

Personally, I am concerned about adding things like acetone or water. Castor is much safer.

My thoughts anyway.

Tom