[RainyEyes]

I'm looking for some insight as to why an AEG would lose FPS due to the hop-up unit. Both guns are classic army with different gearboxes:

I have 2 gearboxes, one TM and one CA.
Both shoot 320+ FPS on hop up unit A.
Both gearboxes shoot 220-260 FPS on hop up unit B.

I've checked the hop up rubber; nothing is torn and bb's do not double feed.
I've checked the distance between the hop-up unit and the gearbox; it fits snug with no play.

Symptom: no matter what spring i change it to (m90-m110) it will always consistently shoot 260 FPS or lower. I've also tried swapping cylinders, cylinder heads, etc. Air compression is not an issue within the gearbox.

So far i've isolated the problem to the hop up unit itself after swapping the hop up bucking and barrels as well.

Interdasting feature: If i turn the gun upside down, insert a mag, remove the mag, and put my thumb over the bb feed hole and shoot it, the bb's will shoot 300+ fps and gradually drop after the 1st shot. I can feel the first shot suck my thumb and air into the hop up unit. The FPS stays consistently below 250 with just the magazine feeding into the gun.

What does this mean, and what can i do to fix it aside from replace it? Is it possible that the nozzle is too short?

This is quite a head scratcher and i'm trying to get some theories on what went wrong and why replacing the hop-up unit seems to fix the issue.