Classic Valve Shootdown

[WarHamster]

Hey guys. I recently picked up a classic valved minimag for $50 heavily used. The guy brought it into our shop looking to get store credit for it. The store was going to give him $30 or $40 towards a proto rail, and I just gave him $50 for it on the spot.

Anyways, I took it home, cleaned it up, and gassed it up. I have it running on HPA with a full tank, and it's getting horrible shoot down. Over my chrono if I try to fire it faster then around once per second the velocity drops off by 30~40fps per shot (For example three shots fired in one to one and half seconds will read something like 280, 240, 210).

I haven't actually cracked the valve open yet to clean it as I'm not terribly experience with automags (though, I'm plenty technically inclined, just my experience is more towards autocockers). All I've done is striped it down and cleaned the body, rail, trigger, etc. Do you guys have any suggestions for what could be causing the shootdown? Is it likely that the valve needs to be rebuilt or does it just need a good cleaning?

Thanks for your help.

[Ruler_Mark]

flip the spring pack, to get to it unsrew the velocity adjuster

[WarHamster]

I opened up the back of the valve, and I can't flip the spring pack. it has a washer on one end that doesn't fit into the velocity adjuster.

[Ruler_Mark]

http://akaog.org/forums/gallery/1_20_09_08_2_05_19.PNG

part #329

[WarHamster]

Yeah, I actually pulled up that exact diagram when I was going to tear it down. I took some photos to show you what I mean about mine...

[ManInBlack]

If it's been heavily used you're going to want to get a parts kit for it anyway, so do that first.

[Spider-TW]

That power tube spacer doesn't belong in there (the brass ring). Maybe its a kluge for the velocity adjuster. While you have that off, stick a ball wrench or something similar in the piston in there and pull it out and clean it. Clean the inside walls of the reg too. Oil and replace. If the pack is oily, that's good. It can rust.

I would think that kind of shootdown is from the on/off being choked or the reg piston being terribly sticky. I would see if you can get rid of that power tube spacer (if the adjuster will work without it). You may need a new spring pack if the guy was trying to get more pressure out of it.

With a used mag you really can't tell where the problem is until you clean it up. They can be plain ugly inside, but I haven't heard of one that couldn't get going again.

[smoke63b]

The good thing about mags is that there isn't much there that can go wrong that a good old parts kit can't fix unless someone did some crazy sawing or grinding on something lol.

Good luck.

[secretweaponevan]

Probably just crapped up.

Especially look for gunk in the reg piston, or teflon tape or other debris restricting some gas path.

[BiNumber3]

Have u tried the gun without that spacer? spring may be unable to move as it should, ive played around with some regs n if the spring doesn't "spring", u get no pressure, tho seems backwards since more compact springs usually mean more air...
sry dunno the mechanics of regs so if im wrong just roll yer eyes at me:P

[Coralis]

That spring pack is wore down pretty good and the spacer is there to give it extra "push" because the combination of the wore spring pack and thumb adjuster (alot of thumb adjusters were cut deeper than the stock adjusters) will not allow the mag to shoot at a high enough velocity.

[michbich]

Is your sear rod adjusted? Their should be about 1MM between the trigger and rod when it's aired.

[WarHamster]

Well I feel immensely stupid now. I took it into work on Sunday (I work at a PB shop on Sundays for fun) to work on it with my buddy / co-worker. We systematically tested EVERY component in the valve. We found in addition to adding a velocity adjuster the guy had gone crazy with other upgrades. It had a custom on/off we'd never seen before, an ANS phase II bolt, custom internals for the firing tube (a weird spring instead of a spacer), and an 8-hole mod in the valve back. We took a working classic valve out of one of our rentals and slowly swapped parts one at a time to try to fix it and nothing worked.

After about an hour we'd tested EVERY piece in the valve including the valve assembly front & back. Then we moved onto the rail (in case it had been warped), the air fittings, and the tank to make sure we weren't missing anything non-valve related that could be causing it. Finally we gave up on it for the day. Then later in the afternoon we threw it into the classic mag (who's valve had been donating parts for our tests) on a whim. SUPRISE!!! It worked flawlessly. We looked at it for a second then it hit us. The ONLY part that was different that we hadn't tested was my CP De-gassing ASA. I had taken it off a working gun and hadn't thought much about it since it had been working fine on my other marker.

At some point the degassing ASA had busted and was SEVERELY restricting airflow. This was why the mag took forever to recharge! Anyways, we threw the valve back into my mag, threw on a new ASA and it worked great!

I guess the moral of the story is that the likely cause of your problem is usually staring you in the face and you're too tied up with smaller issues to notice it!!! But hey at least I have a working mag now. So anyways; I paid $50 for the mag, threw on roughly $70 worth of spare parts I had laying around, and spent about 3 or 4 hours cleaning it and fixing it up with a little TLC. The result was THIS:




Thanks for all the help you guys contributed towards getting her fixed up. I'm sorry I had to have wasted all your time!

[BiNumber3]

so did u leave the valve as u got it as far as spacer placement and that wierd spring u found?

[WarHamster]

Yeah, we left the spacer in the velocity adjuster and the spring in the firing tube (where a spacer ought to go) and it's working fine. Pretty consistent +/- 5~10 or so over the chrono, and it seems to be working flawlessly.