[yuhaoyang]

A modify S120+ is similar to maybe a M130-M140. I was getting rediculous fps with a S110+ already(S110+ was roughly like a M120)
try to swap out the spring for something else, such as a systema M120 or M110.
Other than that, recheck your shimming.

Also, one problem I encountered when I installed new gears was the tappet plate caught on something. With the gearbox closed try to push the nozzle back with your fingers, some part may be caught on some other part.