Huge fps drop/ air leak??

[Xtatic]

Hey guys,

Been trying to figure out this 90+ fps drop in my gun and cant find a solution anywhere. I installed an ASCU Mosfet, Mask Piston Head and Cylinder Head, and my fps went from 400 (0.20) to 290 (0.20).

Ive been using a Modify Hopup unit, and when i go back to the stock hopup unit, the fps goes up to 320ish.

Im thinking its a air leak where the nozzle meets the hopup unit??? Any ideas??

Its a VFC SR16.
Prometheus Tightbore Inner Barrel (370ishmm) <-- i have a silencer on the gun.
Stock V2 Shell.
Stock Gears
Modify Type 1 Enhanced Cylinder (300-450mm)
Modify Piston
Mask Piston Head
Mask Cylinder Head
Modify Air Nozzle
Modify Accurate Hop Up
ASCU Mosfet (rear wired)
Only using Magpul PTS 120 Mags

I tested compression in the cylinder and i get perfect compression. Tested compression with the nozzle on the cylinder head and get perfect (afaik) compression as well.

I noticed a rip in my hopup rubber, replaced it and fps is still low.

Thanks for your help guys.

[lurkingknight]

you still have your old piston head? try putting that o-ring on your new pistonhead.

You could also be losing some air on the new cylinderhead seals.. try wrapping some teflon tape around the cylinderhead to seal it better.

you might need to massage the hopup rubber around inside the hopup unit... sometimes it's not sitting perfectly round or far back enough, which is probably why when you switched back to original it seemed to seal better.

Also try blowing into your hopup while plugging the holes and barrel, you might have a leak between the unit, rubber and barrel. a bit of teflon tape around the end of the rubber towards the front can help seal it.

[MaciekA]

Originally Posted by Xtatic

Hey guys,

Been trying to figure out this 90+ fps drop in my gun and cant find a solution anywhere. I installed an ASCU Mosfet, Mask Piston Head and Cylinder Head, and my fps went from 400 (0.20) to 290 (0.20).

Ive been using a Modify Hopup unit, and when i go back to the stock hopup unit, the fps goes up to 320ish.

Im thinking its a air leak where the nozzle meets the hopup unit??? Any ideas??

Its a VFC SR16.
Prometheus Tightbore Inner Barrel (370ishmm) <-- i have a silencer on the gun.
Stock V2 Shell.
Stock Gears
Modify Type 1 Enhanced Cylinder (300-450mm)
Modify Piston
Mask Piston Head
Mask Cylinder Head
Modify Air Nozzle
Modify Accurate Hop Up
ASCU Mosfet (rear wired)
Only using Magpul PTS 120 Mags

I tested compression in the cylinder and i get perfect compression. Tested compression with the nozzle on the cylinder head and get perfect (afaik) compression as well.

I noticed a rip in my hopup rubber, replaced it and fps is still low.

Thanks for your help guys.

You've tested some parts of the compression system but not all parts of the compression system. Given the parts you've already tested, the primary area of suspicion now is the nozzle-bucking "kiss".

The first thing you need to do is get your air nozzle to the position where it is completely forward. This can be done two ways:

1) Reassemble your gearbox, disassembly will force you to relax the gears and for the tappet/nozzle to be completely forward.
2) Release the bevel / anti-reversal lock (if you know how to do this trick that is) so that the gears can return to home position and allow the tappet/nozzle to sit in the forward position.

It is absolutely key that you truly release the nozzle to forward position for this test. Once you get it there, your goal is now two things:

1) Take your sealed gearbox in one hand, take your hopup and barrel (which should be assembled together) in the other hand. Now shove the gearbox into the hopup unit so that the nozzle is right in there. You want to press the hopup unit TIGHT against the gearbox. Look into the hopup feeder hole. You should see the nozzle coming in to "kiss" the bucking on the lips. Are they pressing each other? They should be. When you slightly let go, is the bucking (ever so slightly) pushing the gearbox away? It should be. This is the most important seal in your system. I rarely see piston o-rings or cylinder head seals causing nearly as much leakage as the bucking-nozzle seal. Do this test and ensure everything is good. If not, try another bucking from a different brand. After changing, make sure that you can still put a BB in the hopup unit and tap it through the chamber manually without too much force (almost no force should be necessary -- tap it lightly with a screw driver or something).

2) Now that you have your kiss established and you KNOW that the seal is good when the nozzle is in the forward-most position, your one mission in life is to ensure you can replicate this situation when the gun is assembled back together again. Do whatever is necessary to push the hopup unit back against the gearbox. This job is usually done with a spring on the front of the hopup unit. Did you lose it? Is it crappy? Sometimes some hopup units aren't ideal for some guns and you'll need to push that hopup back against the gearbox using some other method. My favourite is to wrap the spot where the barrel and hopup unit meet (the spot with the little gold ring) with Teflon tape, a couple wraps at a time, until it becomes more difficult to re-assemble the gun -- not insanely tight, but tight enough that you have to push the gearbox against the hopup unit with some good force when putting everything back together. That will ensure that the "kiss" can't be disrupted by a mag or by any other forces.

Again, this is the most important seal in your system, so it's worth spending some time on it. Hope that helps.

[Xtatic]

Alright so i took the gun apart again, and tested each section for perfect compression.

Piston ----> Cylinder Head = perfect compression.

Piston ----> Cylinder Head ----> Air Nozzle = Leaky Leaky......!!!

My Air Nozzle has the O-Ring inside. How do I go about fixing this??

Keep in mind i have 2 different nozzles, both with O Ring inside, and both have the same leak issue. Have also tried with a different Cylinder Head. Same leak issue at the nozzle.

I saw a video about putting teflon tape (very thin piece) around the Cylinder part of the cylinder head, but that doesnt seem like a good thing to do.....

[MaciekA]

Is this thing on?

[Stealth]

lol...

[Xtatic]

It's on. I appreciate the response. As far as the "kiss" goes, it's good. The air leak is coming out the back of the nozzle, not where it butts up with the hopup (that part is good.

[Trev140_0]

Listen to Maciek.

Re read what he wrote... after that, re read it again.

He covers this front to back. He should send you an invoice for this.

[MaciekA]

Originally Posted by Xtatic

It's on. I appreciate the response. As far as the "kiss" goes, it's good. The air leak is coming out the back of the nozzle, not where it butts up with the hopup (that part is good.

Yeah, sometimes you'll get a lot of seal here but most guns that I work with don't perform that awesomely on the manual nozzle-hold-on-cylinder-head leak test. Even the ones with fat o-rings that are hard to move along the cylinder tube will hiss out hair.

For whatever reason, these setups still fire with really good compression once everything comes together. I'd love for someone to actually show me a nozzle that can stop all the airflow with just your right finger covering it while your left arm pushes the piston into the cylinder -- during a manual test. It doesn't seem to matter that much in practice once you've got the basic o-ring contact area greased up and touching the cylinder head. Much like the expansion of the o-ring on the piston head, it may be only observable/measurable while the system is actually in motion and the parts are moving fast.

Troubleshooting problems like these is why I like to have a big spare parts bin available with alternate heads and nozzles and buckings to try.

You should also consider that while you may have convinced yourself the problem is that the cylinder head / nozzle mating, it could also actually be your bucking leaking (dental floss mod), or piston binding, or any other number of things... Also, the kiss may be good while the system is part, but once you've got a mag in, it may be separated. This is why I generally try to push the hopup unit against the gearbox so I can remove that as a factor no matter what.

[Xtatic]

So this last week ive become OCD about getting a perfect seal in my gun. I double checked everything. Put new O-Rings in. Im finally getting perfect compression with air nozzle attached to the cylinder.
I shaved a mm off the front of my tappet plate to ensure the nozzle would butt up nicely with the bucking. It does. I also did the floss mod, and teflon tape over the hopup unit/ barrel.

I was so excited...i take it to my chrono......275 fps.....!!!!! What the EFF!!!!!

So i shoot like several more rounds through it.
Shot # 2 ---- 275
Shot # 3 ---- 275
Shot # 4 ---- 275
Shot # 5 ---- 278
Shot # 6 ---- 275
Shot # 7 ---- 275

So im thinking, the fps is damn low, but its never been so consistent like that. So im starting to think maybe my chrono has downs. I grab my mp5, which I KNOW shoots 350-365 quite consistently, and i havent tinkered with it at all.

I shoot it through my chrono......245 fps!!!

So my conclusion is my chrono is a piece of hud, and its time for an early Christmas gift. A nice chrono ( any suggestions!!??)

Regardless, thank you so much for the help guys, i really appreciate the ASC community.

[lurkingknight]

xcortech is what I have... though my and my teammates' guns chronoed 385/390 and at nightfall where they FR was using their xcortech, all our guns chronoed another 10fps on top of that, so calibration could be a bit wonky, a lot of guys were saying their guns were coming in a bit hotter than normal. But then again, I've read that the madbull chronos can have that kind of discrepancy as well.

[Spike]

Xcortech is what I use too... I've hear a lot of crappy reviews of the madbull ones.

[Styrak]

What kind of chrono? Maybe it needs to be cleaned out. BB splinters/dust/etc can get on the sensors.

[Xtatic]

It's a Swiss Arms chrono