Double feeds - trying to isolate the problem

[Conker]

Hey, hope everyone is enjoying their caturday night

On my gun I'm using 2 uppers, and I'm faced with two problems. It's shooting pretty low so I'm looking to upgrade the spring, but before doing so I want to check if there's any other problems I should take care of while I'm "playing" in the gearbox... the piston head will probably be changed soon too.

Short (10.5) barrel, working perfectly (I choose lower cost parts as precision is a bit less of a matter for CQB than for my outdoor upper):
- Vanaras tighbore
- Element hopup
- Prometheuse soft rubber

Long (14.5) barrel, missfeeding:
- KM tightbore
- King Arms hopup
- Prometheus soft rubber
- SCS nub

Now... I tested both barrels in my friend's gun, and they both feed flawlessly. Short shoots 288fps, long shoots 315fps. I assumne that's a reasonable difference due to the barrel lenght.

In my M4, short shoots 240fps (+/-2), feeds perfectly. Long shoots... a whopping 140fps (+/-15), and misfeeds most of the time!

Obviously the long barrel is seriously leaking, but is it possible that BBs jams in the barrel as the ~135fps isn't enough to push them out? I'm using .25 Bastards. If that's the case, it'll help narrow down the causes of the trouble.

Trying the barrel with one upper or the other doesn't change anything.

Thanks,
Conker

[HKGhost]

it could be your nozzle not making a good seal to the hop up or it's cracked. Open her up and check that, also check if it's the correct length.

[Conker]

Nozzle looks good, and it feeds great with the Element hopup.

[Matt 'Maverick' Watts]

That's a pretty significant decrease in velocity between his lower and yours. Have you compared the difference in parts between the 2 lowers to see where the differences are and what could affect at least the velocity differences?

[HKGhost]

Originally Posted by Conker

Nozzle looks good, and it feeds great with the Element hopup.

then it may be the KA chamber itself.

[Conker]

For the velocirty difference between his lower and mine it's probably just the internal components, that's not the matter here.

Originally Posted by Conker

Obviously the long barrel is seriously leaking, but is it possible that BBs jams in the barrel as the ~135fps isn't enough to push them out? I'm using .25 Bastards. If that's the case, it'll help narrow down the causes of the trouble.

Right now, THIS ^^ is what I'm wondering about.

[Styrak]

New hopup rubber. The edges where it contacts the nozzle and makes a seal are probably worn/shitty.
Do it.

[juicy]

I can't seem to get a PCS and a KA rubber to work together in my G3. Then again, I haven't shaved the nub inside the rubber down as is recommended in many different forums in regards to using an SCS (or a PCS for that matter) with a hop up rubber that has some sort of integral nub. With this set up, all I get is lots and lots of jams.

If I swap back to the old TM rubber that I've been using before, no problems. Both a PDI 05 and the current Prom TBB's work fine for me. From my grand total of 30 seconds of looking into it, I think I was supposed to shave off a bit of the nub from my KA rubber before expecting it to feed in a normal fashion reliably - have you done this already?

If so... wait, if so, it just means it doesn't help. Dammit. Never mind. Blah my mind just stopped operating.

[Phil_Black]

Just swithch your kA hopup with yoir element.

I just hate ka hops they suk elements are the best and the price can't be beat.

As far as barrel are concerned i woud switch for madbull as they are cheap and do e real good job.

Some people may argue that they are not good but from the experience this combo is doing great so far.

I got this in a cqb, m4 length, spr, m16 and results are excellent.

[AngelusNex]

one thing i find that seems to solve allot of problems is replacing the tappet plate. When it's been worn down on me my guns have done all sorts of crazy stuff.

[audi_bhoy]

Yeah you might want to change your hop-up units between the 2 set-up and see what you get from there.

[CDN_Stalker]

Doublefeeds or loss in velocity both point to the hop up unit/rubber. Some units rely on a certain type of nozzle, aka. a square tipped one and not a tapered tipped one. Also, double feeds are purely the unit itself in relation to the bucking and where' it's placed by the barrel. If your retaining clip is cracked or not seated well enough, the bucking will shift forward with the barrel and cause two BBs to load instead of just one, which will greatly lower your velocity as well.

Nutshell, check the above two I outlined and see if any of that makes sense, or points your attention to somewhere you didn't think of. I've fixed many a gun with the same problems and found the above two causes in most cases.

[Conker]

I'll check again with my hopup. Meanwhile the spring was weak anyways so I'm swapping it, but comparing my two nozzles they have a significantly different lenght...

The one I had installed is 19,67mm while the other I have is 21,15mm... Not sure if I'd be better swapping it.

Otherwise I checked the cylinder's compression and it looks good. The tappet plate is quite a bit flexible, but it looks good.

--

EDIT: I took the "empty" half of the gearbox and installed my tappet plate & the nozzle on it. I then put in in forward position... and apparently, the shorter one doesn't go all the way to the front. That's probably the cause of the airleak! I'll reassemble everything and check how that goes.

[Conker]

Reassembled it. And... it works! I installed the longuer nozzle. It's consistent at +/-1 FPS, and the longuer barrel now DOES shoot 315 while the short shoots 290.

I've put in the wrong spring (SP100 instead of SP110) but it's a matter of 5 minutes now that everything works.

That's weird, since I removed that nozzle as I had problems with it... But whatever, now it works XD