FPS lowered

[RaisinBran]

I bought a Prometheus M90 spring for my ICS and I got a perfect CQB velocity but then when I switched back to my stock spring the FPS was lower than before, now I can only make dents in cans and takes around 3 shots in the same place to penetrate.

I checked the air nozzle and no leaks and I switched back to my Prometheus, same as the first time i tested with the m90, is it just the stock spring that sucks?

[CDN_Stalker]

If using non-linear springs, do you make sure to put the end with the tighter coils to the rear of the mechbox (onto the spring guide)? You can lose up to 30fps by putting a spring in backwards.