Part throttle surge can often be fixed with a bypass valve setting, like a weaker spring so that the BPV doesn't fully close until higher pressure. What is your boost level when it happens?

 

I think you mean a stiffer spring.

Or in the turbo smart BOV case it has a pre load setting you can adjust, Just FYI.

 

For partial throttle surging you want the BOV or BPV fully closing with more manifold pressure (closest to 0psi relative pressure) so a lighter spring and/or less preload is better.

Since the SC is always spinning, driving at part throttle at high RPM (high impeller speed) the BOV or BPV needs to be at least partially open or else you can have these low throttle surge problems.

There is a link to the turbosmart spring chart on this page:
https://www.s2ki.com/forums/s2000-fo...werks-1175850/