Disco is firing in full auto! Need help.

[Sqrl Klr]

First off my thanks to Darrel for the latest updated version of my disco with bed liner on the stock for an improved grip and look since the textured finish goes better with the stippling than the previous smooth paint job did. He put silver lettering on the breech to better match with the silver lettering on the scope in replacing the previous gold color lettering. The two transfer port seals were replaced since they were leaking. From all the hard cocking due to the previously used .047" heavy spring with the power adjuster set to high power near coil bind the tube's breech screw holes were stripped causing a loose breech hence cocking issues and a lot of fliers off target. So Darrel tapped the tube holes as well as the breech holes for added support for the screws to prevent the breech from coming loose again.

A pic of the rifle(Click pics to enlarge).
 


This one shows how well the silver on the breech and scope matches each other.


Here's a good look at the stock to see how the painted texture better blends in with the stippling.



Darrel said it fired in full auto once so he turned out the rear adjustment screw which appeared to fix the issue. I fired it 13 times while sighting in the scope then the final shot was a full auto burst lasting over 1 second. That lowered the psi from 1,400 to 800 with what sounded like around 25 shots in the burst. My current power setting is using a medium .041" spring with the power adjuster turned out to 15/64" gap for medium-low power likely around 17fpe but I haven't verified with the chrony yet. Normally it would take 12 shots to get from 1,400 to 800psi. Any ideas on the cause and how to fix the issue?

[Oldnoob]

WOW. Um, striker too heavy + weak sear spring??  Maybe  a need for a marauder debouncer kit? maybe spring too long?  Maybe upgraded spring too strong with power adjuster?

[Sqrl Klr]

Thanks noob. The .041" spring is shorter than the previous .041" spring I had also used after removing the .047" heavy spring. I didn't have this issue before with this spring so it can't be too long or strong. This didn't happen until after Darrel worked on it so surely he caused it somehow. lol

Just kidding Darrel. I don't know if the sear spring is weak or not but I suspect not because it had been replaced within a year without this issue occurring until now.

[Oldnoob]

Could the striker be getting caught on the screw thread mods? one way to tell is to pull the striker and check for marks on the top of the striker.

[Sqrl Klr]

I suspect no since the same screws didn't cause that beforehand. He would have cleaned the debris from the tapping and made sure no metal shavings were intruding the tube.

[Big Bore Bart]

  It looks like you have a Mrod trigger group, does it have a spacer between the trigger and the tube?     If not, the sear may be broken.     

[Sqrl Klr]

Yep my spacer from Lloyd is the first one he ever made for the discos with mrod trigger assys I believe since the cocking issue hadn't been discovered beforehand to be due from the spacing difference on it compared to the mrod until Lloyd discovered it with this disco and resolved it with this shim where he then started making more to sell since he knew others had the problem as well.

[Oldnoob]

valve spring or stem going bad?

[Sqrl Klr]

I wouldn't suspect so since it's only maybe 2yrs old. Btw I discovered that the shim is indeed not reinstalled so that's likely the issue and thanks Bart for the mention of it that made me check for it. I'll get it shipped to me so I can install it but in the mean time I'm wondering if adjusting the trigger screw would help.

[Oldnoob]

I wouldn't suspect so since it's only maybe 2yrs old. Btw I discovered that the shim is indeed not reinstalled so that's likely the issue. I'll get it shipped to me so I can install it but in the mean time I'm wondering if adjusting the trigger screw would help.

glad you found it.

[Sqrl Klr]

I need to add that I don't assume the shim will necessarily fix the problem since this issue never happened before I had the shim.

[Sqrl Klr]

Well the shim is installed after all. This one seems more thin where it doesn’t show between the breech and tube like it used to where I'm wondering if it's another shim than the one that was on it.

Anyway I adjusted the trigger screw as far low and high as I could get it where the safety would work right with the gun still firing completely but it still fired full auto regardless. I'm hearing a lot of hammer bouncing even with single shots that sound like 5 shots in 1. Now I’m suspecting the issue is due to something internal not allowing the valve to close all the way at times.

[Big Bore Bart]

   The shim goes between the trigger and the tube.    I ran across a post by Bob (rsterne) about this behavior.   Check the stem-valve fit.  (where the stem goes through the back of the valve)  A loose fit will let the air semi-cock the hammer.  Also check the vent hole at the bottom of the tube, between the hammer and the back of the valve.   Someone had a Mrod stem in a Disco valve, or vice-versa.

[rsterne]

Make sure the vent hole in the bottom of the tube at the back of the valve is not blocked by the valve.... If it is, grind a notch in the back lower corner of the valve to allow air to escape from between the hammer and valve....

Bob

[Sqrl Klr]

It has the vent thanks to Lloyd when he worked on the valve.

[lloyd-ss]

Very perplexing.  It seems to fire ok at higher pressures, but you get severe hammer bounce starting at 1400 psi.  I don't think the trigger spacer or the trigger sear have anything to do with this, but I could be wrong.  Bart and Bob have already suggested high pressure air blow-by from the valve, and a cushion of atmospheric air getting trapped in front of the hammer.  Those are good suggestions. 
Is the gun any harder to cock than before?  Could there possibly be a tubular spring shim in the hammer that is boosting the hammer spring force to cause bounce at pressures higher than normal? 
Have you taken the trigger group off to verify that the vent hole in the tube at the back of the valve isn't blocked? This is  a tough one.

[rloftus]

I would suggest that Bob's experience with the auto firing sequence must be the problem.  In modifying my Discovery, I found a huge and alarming play around  the valve stem shaft which I fixed by machining a brass insert to fit the valve housing.  See this post:

http://www.gatewaytoairguns.org/GTA/index.php?topic=63136.msg604321#msg604321

I really wonder if the proper relief port was placed in the valve housing to relieve the back pressure.  If so, it might help if the rear valve face was slightly relieved up to the pin itself, and then down to the lower relief port.  I wonder too if there might have been a modification to the valve closing spring that could have contributed to the problem.

I think properly bushing the pin would solve all such auto firing problems, but it is a bit hard to do.



 

[lloyd-ss]

The old discovery valves had the o-ring (18E) shown in the Crosman exploded view, but they haven't been used for a long time.  I have carefully peened the aluminum of the valve at both ends of the valve stem hole and then forced the valve stem  thru the hole to size it.  A temporary fix for the blow-by.

[Sqrl Klr]

Lloyd yes it is harder to cock than before which is why I suspected the sear needed to be adjusted lower but that did not help. I do not know if the valve vent is being blocked by something since I have not been able to take it apart yet but I'll look. There is no spring shim in the hammer causing the issue. I wouldn't believe the valve would already need to be fixed to prevent blow by since it hasn't even been a year since Lloyd modified it and I haven't shot it a lot in that time.

[Oldnoob]

I know this may not be connected to your problem Klr,, but i can't but wonder about the valve spring in the Discoverys.
I know that with my Disco,, i can't begin to pump it up from empty without having it cocked, because apparently with 0 pressure in it, the hammer/striker puts enough pressure on the valve to keep it open. which makes makes me wonder if the valve spring is strong enough. Also it makes me wonder if possibly the valve spring in the Marauder may be stronger. One possible senario that I wonder about, would be if someone put an inlarged transfer port and a heavy hammer spring in a discovery and add to that maybe a weakened valve spring,, the chances of the valve being held open increase.