Need a bit more specific information....
As others have described there are a number of different gears, each with their own name. It'll help diagnose problems.
More to the point is "Changed it but still doesn't work" is the most important. What was changed?...and more description of what "doesn't work" is needed. Plain language laymans description is fine....as long as there's enough detail.
For example...there's a world of difference between...
"it shoots but makes a horrible whining sound"...and..."when I pull the trigger nothing happens"...and..."the gun cycles but no BB comes out"...etc...
Re. FPS...if you want to be sure that you're under 350fps with 0.20g BBs then stick to a 100% or M90 spring (a Modify S90+ is a good spring). In that mechbox it's likely to shoot 310-340fps. You'd still need to chrony to be sure.
Re. who's doing the upgrades/downgrades/work...you might want to find someone else to help you because any decent gun doc would have:
1) chronied the gun to tell you what it was shooting
2) picked an appropriate spring so it'd be in the right range
3) set the motor height properly and made sure it was going to stay there
Re. stuff breaking....happens all the time, sometimes suddenly. A good setup will simply last longer than a poorly assembled setup.