Hissing after pumping, velocity issues.

[Headshotted]

Hey guys! I think this is the right place to post it, but, running into some issues with this LB I picked up recently, and I'm not sure if it's due to a tank with too high output pressure that might have damaged a spring which I guess means time for a rebuild, or, whatever. So here's what's going on

1. Air up a brand new HK army tank.
2. Install into marker via rear, air rushes down the barrel until cocked.
3. After firing, air rushes back out until cocked again, so gotta be quick like lightning to stop it happening.

I noticed it most when I had the tank at a full 3k, and when the tank was dropping below 1k. It did chrono well for the first few games, and eventually it just became routine and I would find it hissing at the end of the tank and at the very top of the tank. I can attempt to get video, the P/O suggested it might be a cupseal issues which means... well I don't know what it means so I'm turning to y'all, the experts. I have the tools to tear it apart aside from a sheridan valve tool, I do need to pick one up. Will show the video later today/this week. Also, is this normal bolt extension?

[AnarchicArctic]

Just a quick spitball here, sounds like your valve pin is being depressed at rest until you cock. I’d guess your valve spring is cooked, or your mainspring is too long

[Jordan]

Bolt is normal, the back retracts to allow the ball bearings in the front half to move in during the pump stroke.

I agree with Mr. Arctic - sounds like the springing is jacked. I suspect someone shimmed the main spring to try to get more velocity on HPA without doing any other work - you'll probably find something jammed behind the spring, like a ball bearing.

[XEMON]

Or your tank pressure is too low ... What's your shooting velocity?

[Shaftski]

Does it feed paint OK? If so my money is on something with the valve, probably valve spring. Too much shim or spring length usually stops the bolt/hammer from moving back far enough interfering with paint feeding.

[Jordan]

Oooh, good call. Didn't think of that...

[Headshotted]

I don't know what the tank output pressure is, i just grabbed a 13 ci from HK being sold by Zephyrpaintball on the ebays. Edit - According to Amazon, ebay, the output is 800 PSI

I'm getting the occasional double feed, and sometimes the pump action will stop short and it won't feed correctly, like it's jammed. I did notice that where my tanks gauge is in directly inline where the knurled bit on the back end is, and it would catch once in a while so I figured that might be that and solveable with a quick filing on the gauge, but, ocassionally it would feel like it was jammed unless I turned it upside down and gave it some wiggles.

It seemed to be chronoing in the 280's, but I would get visible drop off between rounds if it was 5 in a row. I don't have a hand held chrono or a place to really test it, sadly, closest field is across town from me and no back yard. With all the information above and all new information, what would be my best course of action? It was palmerized according to p/o, so, tips, suggestions, parts to look for would be useul. I have tools, sans a valve tool. I'm sure there's marker breakdown on here I should look at.

I should also add that after the first game of the day I pretty much stopped having it dumping air, and it seemed to work best when pumping from close to perfect horizontal, or at least do it not as much so it seemed, and then would just do it at 3k psi and sub 1k respectively.

I've got some models to finish painting then I can clear off my work desk and start the teardown process. Is there anything overt I should be looking for? I'm not opposed to sending it out for some teching as well, but, if I can fix it with everyone's help I would prefer that.

[Headshotted]

Haven't had a chance to tear it down, desk is almost project free so I can do so. Bad video but you'll be able to hear what I mean.

https://youtube.com/shorts/5CYTkppNg70?feature=share

[Headshotted]

Sorry for the annoyances. This is about as far as I can get without a valve tool, haven't had the chance to make one yet. Aside from some rust, everything looks okay, but I'm still looking at diagrams . The valve seems to move okay, it feels a little gritty, so, working on bashing out a tool.


Edit - after cleaning out all the grit and gross, it aired up correctly this time, no hissing, etc. Between shots it's still hissing air down the barrel/breech, and that's about it from what I can tell. I am now out of air for testing, so, back to the drawing board until next weekend..

[MAr]

So the high pressure is NOT the problem. There isn’t enough pressure behind valve to counteract the spring pushing on the hammer. The hammer rests on the valve pin, with enough pressure behind it from a full air tank with 800psi it shouldn’t be an issue.

With CO2 you could back off the velocity adjuster or cut a ring or two off the hammer spring and still get velocity.

[MAr]

Shoot me your address and I can send you a valve tool. I have a few.

[Headshotted]

MAr you are a saint, and I'll fight anyone who says otherwise.

[Headshotted]

Thanks to MAr and his kindness I've finally got it apart. Spring looks pretty gross, and inside the chamber it's kind of dirty so gonna clean it out. Looks like there is a little bit of marring on the face of the pin. Not sure if that's something I can so but look for a new one. Gonna clean up everything and re-assemble as I am out of air to test with.

Edit - found the valve orings. These things are toast! Super hard and dry, and partially pinched

[jeramiej]

Nice work!

[Headshotted]

Couldn't have done it without MAr. Looks like an oring kit is in the future and maybe sourcing some parts, but probably not. The valve's a little, well, marred up on the threads but seems to be okay otherwise.

[Headshotted]

Got a rebuild kit from Xenom on the way, but it looks like the seal on the face of the valve stem is a little marred. Unsure if it's a two part item and is supposed to seperate or I am supposed to dig the oring out.

[XEMON]

If it doesn't have any nicks or crack, it might be OK.
Put a healthy dose of grease on there when you put it back together and give it a shot ...

[Headshotted]

That was kind of my plan. I haven't had a chance to pick up mail, and I've got literally just enough air to test this, so, hopefully.

[Headshotted]

Hokay. I think I have it mostly sorted. Managed to get it to cycle and dry fire about 5 shots before I ran out of air and it did the hissing thing again. Gonna take a look at it one more time, lube, tighten, and call it hopefully good.

Edit - is there any off the shelf pistol grips that'll work on this? My dad's been getting into woodworking lately and I asked him if he would be interesting in making a set of grips, so now I need a set of grips so I can take it to the field this weekend and tune it in.