Systema M120 Mechbox @ 270fps

[Zekk05]

So heres the situation.

I started building a C7A2(A3) a while ago, and put a systema mechbox in.
put in the motor, tightened it properly, pulled trigger and it shoots. great. go to chrono... 270fps.
huh?
as far as I know, m120 springs are supposed to be shooting around... 390!
Either way, the gun had sadistic range, so even with the low FPS I was able to play with it for a few games without issue ... until the red piston broke! score.


A month or two ago, I snatched up another M120 mechbox off of here for a very good price. Got around to fixing it last night with my tech, and it works flawlessly... So, slap it in the gun, pull trigger. and it shoots. Great. Go to chrono... 275 fps. DAMMIT!

The first mechbox was used in conjunction with a guarder hop up and deepfire 6 04 barrel.
The 2nd mechbox was used with a systema mechbox, and systema 6 04 barrel.

Is the 120 spring only rated for 270fps? We put 120 springs in some JG rifles and they fired 420 fps!
Compression is damn near perfect. Cylinder head isnt damaged, nor is the air nozzle (both brand new) hop up is off. what could be goin on here?

[CDN_Stalker]

You have a bad airleak problem, either your hop up rubber isn't installed properly, is ripped, or the nozzle on the two mechboxes isn't compatible with the hop up unit you currently have.

[Flatlander]

Does your hopup unit and inner barrel bounce easily/smoothly in and out of the outter barrel without getting stuck? Did you try the tephlon tape mod for the hopup?

[Zekk05]

rubber was checked, not ripped. lubed up a lot though.

and the airnozzle is systema, as is the hop up unit? its just a standard 1piece metal hop up

Edit: teflon tape mod? no, we have not. do tell!

[Flatlander]

Tephlon tape mod: Wrap tephlon tape (aka plumbers tape) around the hopup rubber and continue down slightly past the rubber onto the inner barrel (I go about 3/4" past). This prevents blow by: air blowing by between the hopup rubber and the inner barrel. Put a bit of oil or grease onto it to slide it into the hopup unit as it's usually a tight fit. I can only get one wrap otherwise the little C clip that holds the barrel will sit too high, causing the hopup/barrel to not bounce freely. I cannot stress enough how important this bouncing is with the hopup!

Me and a buddy were noticing low FPS on our m4's soon after upgrading. I did the tephlon tape to both of ours and it bumped up both about 40-50 fps. Now on some guns it has little to no effect as the rubbers are sitting tight enough on the inner barrels.

[Skladfin]

try prometheus air seal hop up rubber if you have time and money.

I have one and they seal air like a charm

[CDN_Stalker]

Teflon tape won't work in all hop up units/rubber as many are tight enough as it is to fit the rubber into the unit. For the most part, the teflon tape mod is for sniper rifles, but even a few wraps of tape on the rubber would cause ripping and also make the rubber/barrel not seat properly without some difficulty.

[Maverick0]

Originally Posted by Zekk05

So heres the situation.

I started building a C7A2(A3) a while ago, and put a systema mechbox in.
put in the motor, tightened it properly, pulled trigger and it shoots. great. go to chrono... 270fps.
huh?
as far as I know, m120 springs are supposed to be shooting around... 390!
Either way, the gun had sadistic range, so even with the low FPS I was able to play with it for a few games without issue ... until the red piston broke! score.


A month or two ago, I snatched up another M120 mechbox off of here for a very good price. Got around to fixing it last night with my tech, and it works flawlessly... So, slap it in the gun, pull trigger. and it shoots. Great. Go to chrono... 275 fps. DAMMIT!

The first mechbox was used in conjunction with a guarder hop up and deepfire 6 04 barrel.
The 2nd mechbox was used with a systema mechbox, and systema 6 04 barrel.

Is the 120 spring only rated for 270fps? We put 120 springs in some JG rifles and they fired 420 fps!
Compression is damn near perfect. Cylinder head isnt damaged, nor is the air nozzle (both brand new) hop up is off. what could be goin on here?

You really have no luck at all with Systema gearboxes do you?....

It's 2 for 2, so there must be something that's being overlooked on both boxes. I'm no gun tech, so I'm just throwing this out there... don't think it would cause a drastic drop in fps, but is the cylinder matched to the barrel length?

[Zekk05]

yes, the cylinders are matched. that wouldnt cause a 100+fps drop.

will try teflon fix soon. cross fingers...

[Renegade)]

Should have just let me have that box eh man? :P Are you using the same spring for each box, or one new one old.. ect, if your sure theres no airleak, then could just be a bad spring.

[Jugglez]

Check that you are not overtightening the stock screw.
An over-torqued stock screw is going to pull the gearbox back slightly, creating a small gap between the nozzle and hop-up unit, and you'll leak all kinds of air.

The fix is easy, use an appropriate sized washer or nut on the screw to prevent it from being tightened too much.

[Zekk05]

Originally Posted by Jugglez

Check that you are not overtightening the stock screw.
An over-torqued stock screw is going to pull the gearbox back slightly, creating a small gap between the nozzle and hop-up unit, and you'll leak all kinds of air.

The fix is easy, use an appropriate sized washer or nut on the screw to prevent it from being tightened too much.

THAT is very possible.. I never even thought about that.
good call! cant wait to get home and try

Renegade, it has nothing to do with the gearbox. its working just fine. the problem is with the gun its going into. This is proven because Both my m120 gearboxes are showwing the same problem. almost identical.

[Renegade)]

Ah, in that case I second Jugglez motion, and when the gun is together, look up through the magwell and see if the hopup sits tight to the mechbox, and that there is no movement.

[CDN_Stalker]

That's what I said, common denominator is the hop up unit/mechbox interface.

[ILLusion]

What hop up rubber are you using? I've noticed that some over-stiff hop up rubbers will cause velocity drops similar to what's noticed here - it also creates feeding issues.