Low FPS
 

Ok, so I installed a new JG version 2 gearbox into someone's M16 with a Modify S120 spring in it (quick change gearbox, didn't need to open it), and the FPS was only 265. The hop-up was damaged, so I replaced it with a new systema Energy hop-up and tested it again. Same thing.

What's going on here? Is the inner barrel bent or something? Like what would be causing this?

Chrono is an Xcortech 3200, and my KWA M4 clocks in at around 360, so it's not the chrono.

Nozzle/hop up unit mismatch.

Make sure your hop-up unit is pushed right up against the mechbox. Do this by using shims and/or O-rings. The little tiny spring that most guns come with sucks.

Also, check the seal between your hop-up rubber and the nozzle. Do this by re-setting the gearbox so that the nozzle is in its most forward position. Re-assemble the gun, then shine a light down the BB feed tube. Look through the barrel on the muzzle side (make sure there's no battery plugged in and no BB in the chamber), if you can see any sort of light, your seal is not ideal and you should change either your hop-up rubber/unit and/or nozzle.

Unbeknownst to most, there are some "magical pairings" of nozzles and hop-up rubbers and some hop-up rubbers only work with certain nozzles.

Actually it can be any number of things. Check your compression.

I will bet you $50 bucks he has a shitty seal on the "o ring" of the piston.

($50 bucks.... remember this post in a few days if he cracks it open.)

(ps-- blow into the nozzle with your mouth and tell me if there is air rushing out)

Think it would be that? It's a brand new gearbox.

If the piston head is ported, blowing into the nozzle won't ID a bad O-ring, simply due to the amount of air pressure required to inflate the O-ring.....

My brand new G&P box I got the other day had nearly ZERO compression. Had to open ip up, put the o ring over the cylinder (to stretch it) gave it a few good hits with a lighter, and put it back together.

My bet stands.

O ring.

LOL you're betting against CDN_Stalker?
How were you testing the compression?

Lol

I used to test for compression with the mechbox open, no spring, block the nozzle with a finger and move the piston forward quickly to see if the ring would inflate. Sometimes all that's required is a light wipe of silicon grease on the ring to help seal it a bit during use. I CAN happen, but low fps often occurs, in cases such as this build, mismatching brands and types. Some nozzles are tapered at the front, some are squared off. The nozzle has to match the hop up unit or else you'll get low fps.

Now, if the OP can tell us if the nozzles are the same type....... could help, but also know various brands have different tolerances, there isn't really a standard across manufacturers.

Ok so we are not swinging dicks around here:For the record I am not a gun tech, but I sure as hell have rebuilt my share of V2 boxes for me and friends. Too many. This smells like a shit o ring to me.

On the BRAND NEW G&P I JUST got:

I opened the gearbox after I blew into the nozzle with near zero back pressure. Felt like it had no o ring at all. ( btw this is not a ported cylinder)
Then rapidly moved the piston blocking the nozzle. There was ZERO compression in this box. And I mean ZEEE-RO

I stretched the o ring and popped the s130 spring in to get me the 420-430 FPS on the chrono. (gamed it Sat and re checked it Sunday after 2,000 rounds. All good.

Not betting against anyone.

BTW, I have one of those JG boxes too (for my backup) and they are bare bones inside. The piston is red. (all plastic no steel teeth) and I recall there being weak compression from the store on that one too. Not sure about putting in a 120 spring, but worth a shot. But again, remember its all plastic.

As a final thought, keep in mind the box this guy is talking about is BRAND NEW. It has no record of what FPS it is shooting, meaning there is a possibility he has shit compression inside. I know the golden rule, work outside in, but with this exprerience I JUST had and also owning a JG box and being inside it, this very well could be.

That said, close the loop on the hop up end before you open it up.


(oh and Stalker, I was born Nov 30th/71, so my appology for not respecting my elders ;-)

Oh you prick!!!! Five fucking days! Lol, my girlfriend is 12 days younger than me, that's the amount of time she can get away with calling me old!!! :)

All the years I did gun work, there were many dozens of V2 problems and upgrades to do, and the farther away from basic upgrades one gets, the more complications can be had, especially with different brands of parts. Some of the worst ones I had a pile of new parts dropped off with a gun, and a good 15-20hrs of air leak chasing ended up being the final straw in my gun work (especially considering I only charged $40 per gun to open the mechbox). Nowadays, there are even more brands, types, clones, etc. I'm really glad I got out when I did......

EXACTLY!!

I was working on a box the other night and started at 3pm and by 11pm I almost pitched it.

all kidding asside if you would have seen this G&P box compression you would have said wtf?

His JG is missing a good 150FPS right now. Thats a bit much for a hop up.

But then again, you are older and wiser so........ya...... .

Never know, it has happened in the past, especially with a hop up unit requiring a square tip nozzle, only to have a tapered tip one installed as an after thought to the parts order. Could be the piston O-ring as well, I went with my first guess based upon seeing different mechbox (with unknown brand of nozzle) compared to another brand's (stock, then a Systema) hop up unit........ like I said, that rang a bell and I went with it. Also assumed the mechbox had been tested by the assembler before it left the factory, I know asking too much from JG. Lol

Regarding the O-ring issue you found, ya, I'd be stretching it too, as well as trying other rings to see which would give the best compression. Airleaks such to chase!!!

I"m betting one million dollars it's a cracked mechbox.

Already? Was said to be brand new! Lol

Like Vizzini says, "You go back to the beginning".

JG quality control is that bad, eh? Oh wait, is that expecting too much from JG? Is possible! :)

I'm going to try this with my VSR after I drill the end cap out a bit. It should be shooting like 450, but it's more like 330. New o-ring. Lighter. Check.

P.S.
I like boobs.

Cut off the piston brake, common VSR issue, heavy spring, still shoots 330fps. Cut it off and realize what you've been missing (piston brake is there due to Japanese standards of 1 Joule bs, aka, 330fps with a .20g BB). If you don't have a piston brake anymore because you cut it off or totally replaced the piston and the head, disregard. ;)

I love boobs too. Maybe that's why we've been struggling with helpful info for you, easily distracted males........... :D

LOL

Ok, where would said piston brake be?

It's the pin in the center of the piston face.

Cut it flush or leave 4mm if you want a quieter gun.

Pretty sure there's no pin.

Had the cylinder apart the other day. What I think it may have been was the BBs hitting the end cap. Just drilled it out so I can see the edges of the barrel now (obviously taking the barrel out first). Gonna have to re-chrono it.

If you're at the point where you're just sick and tired about taking your whole gun apart to take the mechbox out to take it apart to troubleshoot AGAIN...

Check over the mechbox...check the compression as best you can. Even by hand you should be able to push the piston in fast enough to generate an airseal. Check with the nozzle off...with the nozzle on. A little/slow leak with the nozzle on (in the extended position, not pushed down all the way on the cylinder nozzle) is ok...it's tough to get 100%/super-perfect/airseal everytime without lots of spare parts to sift through. If you've got an issues there...fix that first.

Once you've got the mechbox all buttoned up...put on just the grip, motor/motor plate. Adjust the motor height so it's nice.

Then take the complete hopup/inner out of the gun and fit it to the mechbox. The rim of the hopup unit should seat flush with the opening of the mechbox. Some units are too large and sit away from the mechbox...some are casted badly and don't sit flush either. The old DBoys ones used to be off-center and not fit a variety of shells. The spring on the hopup pushes against a notch in the outer barrel to "force" the hopup unit flush against the mechbox and create a good seal between the nozzle and hopup unit. What you're doing is simulating that out of the gun.

You can quickly rig up a jig by driving a couple of nails into a board...but I just hold it all by hand.

If it's all good and flush, you can drop some BB's into the feed tube (usually takes 4-5) and use light finger pressure to keep the BBs in the feed tube (they'll want to pop out when you shoot....wear googles). Then shoot through a chrony. If you're getting the FPS you expect...then the issue is with the body/barrel/stock/grip setup when assembled. If that's the case...I'd try a different mechbox make.

If you're not getting the FPS you expect...then I'd first try another hopup unit/rubber/inner. And if it was still low, that'd point to a mis-match with the nozzle, I'd open the mechbox again looking at the nozzle/tappet (there won't be much to look at with the cylinder/piston head/cylinder head....since you checked that out already). Since it's hard to physically see an good/bad airseal between the nozzle tip and rubber...I'd just swap the nozzle and test again.

Since you're working with a BNIB mechbox, although it's not unheard of that it could be a dud, I'd be looking straight to the hopup unit and rubber. I'd first swap the hopup rubber with a different make/brand...I'd make sure the hopup was set almost off (just enough to hold the BB from rolling out). If it was still wonky, I'd try a different hopup unit.

Best of luck

.

bring it to tys!

;)

I prefer the "buy a high quality gun" approach, but some people insist on buying shitty low-end guns or mixing and matching parts, and I am one of two people in this city left to fix them.

so did you close in on the source of the problem?

No kidding, eh? Silly us, anticipating the result to see who was correct. Lol

ya-- I was a little interested in this as well.

No, I don't have the gun infront of me right now. Handed it to another tech to see if he could do something before the suggestions were made. Will post if solutions worked when I test them.