why low fps on full auto?

[MrMalkov]

My DBoys AKS74U was shooting 180-200FPS with an M90 I installed in it, I just played with the hopup and got it up to 320-330 FPS.

If you have not tried it already, try the hop up.

[Styrak]

I meant the main spring strength, if it wasn't clear.

I guess I would have to agree with Stealth, in that you have such a high rate of fire that the tappet plate can't keep up. However I'm asking myself why it would then work on semi. Perhaps because it takes more time to torque the motor and complete the first shot from rest. That's my best guess

[DustMagnet]

Originally Posted by Styrak

Spring strength has nothing to do with this. By the time the piston is released, the nozzle has long since returned into the hopup unit/rubber.

Oh yeah...

[Stealth]

Is the low FPS reading consistent?

Another problem may be double-feeding. When you took apart the mechbox and hop-up assembly, it shifted something.

BB gets pushed into chamber, pull trigger, and fires fine on SEMI.

In full auto, sometimes two BBs get pushed into chamber and you're getting inconsistent lower readings.

Try shooting down range at a 100ft target. If your hop-up works fine in SEMI, but some BBs drop to the floor on AUTO, it's a feeding issue.

Solution: Sector delay clip and cut the tappet plate fin to 11mm tall.

[Warlock]

If the problem appeared after the gearbox repair, it has something to do with the gearbox. Your new spring might be problem. I would suggest replacing the spring and put nothing else than a systema M110 or M120. Systema are the most consistent and effective spring for AEG. Also a ROF of 40 is way too high...