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Adjust the accelerator pump: https://www.youtube.com/watch?v=k-Nx5HEzvlYAdvance the timing further. I am running that same engine at 17*BTDC with no knocking ever. Timing is almost a horsepower adjustment: more timing gives you more horsepower, right up until knocking. This is why modern cars have knock sensors -- so they can run the timing right at the edge. If you get knocking, back off two degrees at a time until the knocking goes away.
If less is better, even less could be better still....