VFC Mark 12 FPS loss after spring change
 

I changed the spring of my VFC Mark 12 because I had 415-410 FPS. I switched to a brand new modify S100+ spring, and now I have 295-300 FPS.

Upgrades:
-madbull 6.03 509mm
-SHS CNC cylinder head (new)
-SHS nozzle (new)
-Metal hop up chamber
-S100+ modify spring
-Loonex rubber


Things I have tried :

Change back to old spring = same FPS

Change for an other spring = same FPS

Bought a new set of nozzle + cylinder head = same FPS

The ''kiss'' between the nozzle and the hop up rubber is perfectly sealed

Removed old grease and cleaned it, but back back new grease (just enough, not too much) = same FPS

Checked FPS with an other gun = the problem is not the chrono

Put back old hop up chamber and stock barrel = same FPS

Did the compression test :
Cylinder only = perfect
Extended nozzle = perfect until I push very hard, I can hear a very very small leak.

Did the ''paper check'' (gun placed upside down, small piece of paper over the BB hole in the hop up chamber) = barely moves

Added Teflon tape to the front of the hop up chamber = same FPS
My rubber sits tightly in the hop up chamber, I need to grease it in order to put it in

Changed the hop-up rubber with an new one = same FPS

BBs used :
madbull 0.25g Bio = 300 fps
Canadian tire 0.20g = 280-290 fps

Used an other magazine = same FPS

Remove the brass ring on the barrel = same FPS

Ive checked many posts on many forums, and I tried everything. Nothings works. I am seriously thinking to go to the local church and ask a priest to exorcise my airsoft gun. If you have any suggestions or any tests to perform it would be very appreciated. Thanks and sorry for bad english.

Is the cylinder ported? Did you lube the nozzle/cylinder head interface? You might need to trim the tappet plate spring and maybe trim the tappet plate fin down a little To ensure the nozzle is seating properly, the SHS ones can be kind of tight and it may not be returning fast enough.
Or maybe your chrono has low batteries.

My cylinder is ported and yes I put a small amount of lube and the nozzle move smoothly on the cylinder head. I'll change the batteries to see if it works. Thanks! And for the tappet I don't think there is a problem with it, I have been using it for 2 years and I had no problems. Everything popped out after my spring change.

Stupid question: Did you put the cylinder in backwards?

^^ what he said.

though for a mk12 cylinder going in backwards you'd be getting in the 100s fps.. though you should check anyways.

You are very under volumed for your barrel length. You need a non ported cylinder.

Im sorry guys, I forgot the "not" in my last post, my cylinder is NOT ported.
But once I tried to put it backward, didnt made any difference. I have some wear in it. I can see where the piston stops inside it. But since I have a perfect compression I thought that my cylinder is ok.

if compression components are good, the barrel group has good compression, have you done the tissue test? put the gun together, upsidedown, block the barrel and put some tissue in the magwell over the hopup feed. Shoot the gun... if the tissue flies out then your hopup/nozzle interface is incorrect.

You can fuck this up by overtightening the grip screws or the buffer tube screw (vfcs shouldn't have this issue as the buffer screw doesn't go into the gearbox) Missing body pins can cause misalignment, maybe the gearbox isn't sitting completely lined up with the hop when you put it in the receiver.

Stupid question: is the hop-up disengaged/turned off all the way?

I tried it with a turned off hop, just a little, half and max. I lost a few FPS each time I turned it further.

I did your test, and the tissue flew two feet in the air. What are your suggestions to get rid of this problem ?

Thanks

To be honest, it sounds like your fps is pretty consistent and you've passed all the test so you could just have normal performance with an M100 spring at 300 fps. Maybe it's a lemon spring lol? Try it with an M110 and see if it helps. Mayhaps the seller accidentally switched the Modify M100 with an M90 spring?

Alright, I will try an other spring

I just tried a stock spring who is supposed to shoot around 420 FPS, I had 315 FPS... still 15 FPS increase !

And I just found an other problem : Mid cap wont feed well, only high caps.
The problem is the nozzle, after a semi auto cycle, the nozzle is pulled backward. The nozzle blocks the BB from feeding, he is 1 mm too much extended. I solve this problem but putting electrical tape between the hop up and the gearbox. But now I am back to 300 FPS because of this modification.

It seems like, the more you get after market parts, the more you get problems due to incompatibilities.