Any KWA RM4 ERG Owners?

[Cr0-Magnon]

I've just picked one of these up second-hand and would like to bump the power up by about 30 - 40 FPS. I've accessed the spring but it's much shorter than standard AEG's (around 125mm). 

 

Can I use standard length or do I need to order a special KWA one/cut down a standard? If it's the latter, that's going to make working out which spring to use, a serious case of trial and error!

 

[EvilMonkee]

I have had one and currently have the Cent Arms version, its the latter I am afraid....