The O.S. FS-120 Surpass SP takes a bit of running until it's ready. It won't really "come in" until about five gallons or so of fuel have been run through it.

That said, you have to lean the high-speed needle as far as possible without detonation or RPM drop. With the O.S. pump system, if the idle mixture is too rich, even though you have adjusted it as lean as possible, then the high-speed needle's still too rich.

You cannot get this engine adjusted properly without a tachometer.

This engine was originally designed for F3A Pattern, and users typically used a 20/20 fuel, or something like Cool Power 30% H.P Heli fuel. It was typically propped with an APC 14 x 14, or APC 13.5 x 13.3. RPM at peak was around 10K with the 13.5" prop, lower with the 14".