901 valve lock

[longhunter]

I bought a 901 a while back.
All I've done was to fill the piston head, and change out the o-ring.
Oh, yeah, I removed the lawyer spring as well.
I can only pump it 8 times. 9 times and I get valve lock.
I was getting 10 pumps prior to that.
Any ideas? Could taking out the lawyer spring have that effect?

[FastDave]

When you filled the piston head you are increasing
pressure compared to stock piston.

Higher pressures could cause valve lock.

Example:

8 pumps standard piston might only
need 6-7 to achieve same chamber pressire with
the modded piston.


YMMV

[longhunter]

Could be.  It really has me stumped, though. I should be able to get 11, or 12 pumps before valve lock occurs.

[Ronno6]

Valve lock is one of those things I have trouble getting a grip on.
But, seeing as how the only thing you really changed was filling the dimple in the piston, that has to be the reason.....

H put a trigger kit into a Winnie 1977.
It would routinely and predictably see VL at 8 pumps.
Time and time again.
So, I set it down for a few weeks.........or months, dunna 'member 4 sure.
The next time I picked it up, VL was GONE.......... dumped all the air even up to 15 pumps........

No explanation, but I'm sure witchcraft was involved........

Mr. Trey has some good insight on VL in his 30fpe 1922 thread...... https://www.gatewaytoairguns.org/GTA/index.php?topic=204714.40
Dunno if he is right, but, maybe.............

[pblawler]

For this discussion when I say "valve lock" I mean the gun releases some air but does not fully "dump" the pressure chamber.  Full lock meaning the poppet doesn't budge at all is pretty rare and I have only seen that a few times.

The flat spring removal is not the problem.  The piston mod pressure increase is not the problem.

Yup valve lock in these guns is touchy.  I've seen guns that would shoot 24 pumps for a year start experiencing valve lock at 15 pumps for unknown reasons.  Some causes are too much oil in the gun.  Solution is to dry fire 10 to 20 times.  Removing the poppet can cause valve lock.  Something about installing the poppet can cause valve lock, but repeated shooting usually causes it to go away.  I can't explain that one, other than "break-in" (poppet wearing to VB??).   Sometimes a gun will valve lock when dry firing but not with a pellet.  That is because the back pressure behind the pellet reduces the closing pressure (differential) on the poppet.  So test for valve lock with a pellet.  I do not think your piston mod directly caused it by increasing the pressure.  I have installed 1977 pistons in the 901 and achieved 20 pumps so I doubt your issue is pressure related.  If there was any sanding dust in the gun from the piston mod it could be causing issues with the poppet.  I have had guns that increased pump count before valve lock over time, all I can say about that is that there must be something to the poppet "breaking in" to the valve body.  Even reinstalling the poppet that has been in the gun for years will experience this unless the poppet is clocked to the exact same rotation it was originally when reinstalling.  The last thing I've seen is a gun that while shooting on the range would shoot at 20 pumps every time, but when I left the gun pumped for 10 minutes or more it would valve lock.  I assume the poppet would "mash" into the transfer port (TP) if left pressurized for any length of time, making it more difficult to pull.  By shimming the hammer spring the problem went away and has worked leaving the gun pumped for hours.


Things you can do are pull it apart and CLEAN everything spotless to include pulling the poppet and clean the transfer port, poppet, chamber, etc....  Dry all parts of oil before assembly.  Reassemble and verify the hammer spring is correctly seated on the trigger and hammer.   Reassemble, put 4 drops of oil on the piston sponge and start shooting.  You should be able to get 12 pumps no problem.  Valve lock after 12 might happen but should go away after 40-50 shots to where you can reach 20 pumps.  You will bend the abutment pin past 12 pumps.  Replacing with air or oil hardened drill rod helps. 


Good luck

[longhunter]

I took everything apart and found that the poppet spring is much stiffer than one I had laying around for an 880.
Maybe I'll get ambitious and swap springs.
Not now, though. It's doing 690 over the chrony, with Daisy wadcutters, on 8 pumps.
I'll wait and see if it breaks in, before I do anything else to it.

[Ronno6]

I agree that newer poppet springs are stiffer than previous units.......

[Back_Roads]

 Only experienced valve lock on a fresh reassembly, a few cycles cleared it up, as many as 15 pumps no issues.

[longhunter]

Problem solved.
Pulled the valve apart and replaced the spring with an old style, softer spring.
No more valve lock to 12 pumps.
Getting 740fps average on 10 pumps.
Suits me all to heck.