CA "new style" MP5 hopup issues

[Crunchmeister]

I'm currently having a feed and velocity issue with my MP5 that's got me rather perplexed, so I'm now turning to the more experienced gun docs for some advice on what could be the issue. To see the current situation with it, it's probably a good idea to explain the history of the gun a bit.

Background:

It started life as a JG MP5-J and had has slowly been modified over time. This was a long-term project experiment gun, and it'a been great until the last work I did on it, which is admitedly major surgery.

The last "good" configuration of the gun saw it with a CA metal upper receiver & front end, stock, never-opened JG mechbox, and JG lower receiver. The hopup / barrel were the stock JG, loosely fitted in the CA body. This is a body designed to take the new style metal CA hopup that's adjusted under the bolt cover rather than the external level. However, I used it with the original JG plastic TM-style externally adjusting hopup. The gun fired a consistent 330-340 fps, was very accurate, and had a unbelievable range for such a short gun. It developed a crack on the mechbox near the nozzle at about 35k rounds, but still kept on going without issue until about 60k rounds.

So a few weeks ago, I decided I just couldn't kill this mechbox, and it was time to finally gut the gun and install a new mechbox and the proper CA metal hopup I had bought for it. While the gun was proving reliable, I just didn't trust it with the cracked mechbox. I got Deepfire reinforced 7mm gearbox. and a set of Modify modular gears (torque-up), a Deepfire metal spring guide, and reused all the other stock mechbox parts including the spring, since it shot at the velocity I wanted. The original parts, including the piston and piston head showed absolutely no signs of wear, nor did the tappet plate or nozzle. I tranfered all the wiring and shell parts (except installing a new TM cutoff lever) and parts. Compression tests showed an excellent seal, and the mechbox operated without issue. I installed a 6.03 tightbore and a Guarder clear hopup rubber, mounted the metal hopup into the body, and installed the lower, and got ready to test fire. This is where problems began.

Problem:

The BBs seem to get stuck in the hopup and none would fire. At first, I suspected the hopup rubber was installed wrong, so popped the lower receiver off and slid out the barrel / hopup assembly (gotta love the quick-disassembly CA metal body). Trying to push BBs though the hopup, they seemed to jam. After some messing around, I tried to isntall a proper CA MP5 barrel I had (came with my front end). Some BBs were feeding through, but not well. I would end up with low velocities and jams. I tried different hopup rubbers I had, both used (good condition) and new, with similar results. I've eliminated the mags as a possible issue, since they all give the same result.

I really don't think it's a seal issue inside the mechbox. I think it would be more like a seal issue between the nozzle and the hopup if it's a seal problem. Is there any way I can test this?

Another thing I considered was that because the CA hopup is different from the TM style hopup, should the loading nozzle be the same as a regular MP5, or should it be a different style? I wasn't able to find any info on this, so throwing this out to see if anyone knows.

So that's where I am and I need some help. I hope I didn't bore you too much with the long-ass post, but through I would provide thorough info on the gun and its evolution, as it might shed some light on its current problem.

[coach]

I'm in the same boat as you with velocity issues but haven't had a miss feed yet.

-stock TM box
-Modify modular set (torque up)/s100 spring
-Modify cylinder set w/sorbo cylinder head pad
-Modify tappet/nozzle/piston/bearing spring guide
-Mask piston head w/ sorbo pad
-TM EG700
-Madbull 229mm TB

Shoots well below 200fps. Everything installed should yield a much higher velocity so I too am beginning to wonder in the new style CA metal hopup unit is at fault here. Everything in the box is essentially new or in new condition so there's no reason why it cannot pierce the side of a coke can even.

I'll have to tear it down and try the old hopup again, then if that doesn't work I'm at a loss. Compression test was strongso no leak in the box. It must have to do with the hopup unit itself.

[Crunchmeister]

I'm seriously wondering if it has to do with the nozzle. If the nozzle doesn't seal properly with the hopup, that could be it. Hope someone has an answer for that. I love using the MP5 for CQB. Have 2 CQB games coming up in the next 2 weeks, but it's shelved until I ban get the issue sorted out. And don't see myself having much time to troubleshoot from now till then.

[milehigh]

I have a TM MP5 and I am using the CA metal body and hop-up. I didnt find any issues other than about a 10 fps drop (which wasnt a big deal since the hop-up is much more consistent now). I am using the same TM compatable nozzle. One thing I noticed is that the CA metal hop-up buts up against the front of the gearbox. Perhaps your new gearbox has some larger outter dimensions?

[Jaxxin]

everyone check your tappet plates!

Crunch i have a question for you...

Can you take your mp5 to the point where the pistol grip is attached to the mechbox but you can see the mechbox cycle? (lower reciever, pistol grip, and mechbox all together) Can you see the air nozzle cycling? Do you see it actually pull back and forward with every shot?

If that isnt snapping back and forth with every round then that would be the explination for the weak shots and shots that arent powerful enough to clear the hop up.

Check to make sure the spring is installed on the tappet plate, and that it seats correctly and isnt cracked in half.

[Crunchmeister]

Yeah, my tappet plate is fine. That was one of the first things I checked. Thanks though.

[Jaxxin]

can you see the nozzle cycle back and forth? take out the hopup nub and try, maybe its pushing your hopup down to far causing it to have so much hop-up that a bb cant push by it.

[Crunchmeister]

Yeah, the nozzle moves and is firmly attached to the tappet plate as it should be.

[Skladfin]

you sure you got the right nozzle?

[coach]

Originally Posted by Skladfin

you sure you got the right nozzle?

personally, I tried 2 MP5 specific nozzles. one from the Modify MP5 cylinder set and the other was a Modify MP5 nozzle bought separately.

[Crunchmeister]

Originally Posted by Skladfin

you sure you got the right nozzle?

I used the nozzle that was on the mechbox originally installed in the MP5, so I'm assuming it came with the right nozzle. It worked fine until I installed the CA metal hopup.