Modify-Tech explains the issue of low FPS in AEGs and one cause is can also be the hop-up spring. They present their Tapered Hop-Up Spring in their XTC as a good solution... "There are lots of variables causing AEGs low FPS issue. The air seal is a key point. The hop up spring, though small, accomplishes big as it can pushes the hop up chamber back into the air nozzle to reach a good air seal. When the hop up spring is not strong enough or too short, or you lost it when you rebuild your AEGs, the hop up unit would moves back and forth, so it doesn’t always line up with the magazine, with the hop up moving it may cause loss of air seal, causing lower muzzle velocity.

The Modify XTC‐G1 is used with a tapered spring for the hop up, which puts tension against the hop up and air nozzle to eliminate any play/ wobble that may affect accuracy. It allows the hop up unit together with the inner barrel always line up with the magazine and gearbox, and further reach a good air seal. But be aware of its installing direction. If we install the spring the wrong way, the hop up unit would moves back and forth a little bit, so it couldn’t reach its purpose."