pressure dump

Hi, just want to pick a brain or two. I lent my Bullpup condor to a friend while I was at work. He is an engineer in HVAC and quite a reasonable machinist, so I had no worries. But when I got the gun back it had the habit of dumping air fron 2500 psi. down to 1100 psi. I was only filling to 2500psi. but the first or second shot whoosh down to 1100psi. or so. Why?
I have no preload on the valve from the hammer .050″ clearance a brass 100 gram hammer, lots of inertia. Although the problem existed with an alloy 50 gram hammer. The breech slide is very free to slide, the slide only rides on the valve and barrel breech.
It seems if I have too much stroke on the valve this happens. I have a big valve, a .200 port through the stem, 3 x .125″ slots, the seat is .312″ same as the O.D. of the valve stem. I thought that if I went to a smaller dia. valve seat and a longer duration opening i would get the same velocity.
So new valve this time with a replaceable seat so seat size can be varied with just a seat insert instead of a whole new valve. Seat size .200″ 45 degree seat with a 6-32 socket head bolt to hold the valve head. So far so good 14.7 gr. at 1070 fps. and kodiaks at 975 fps. ho ho now I’m on the right track. Oh, not so fast as soon as I go to brag to my brother-in-law whoosh air dump again, same same. Why Why, my hair is grey enough already.
I notice when the valve has a longer stroke, and the gun is fired empty no pellet it sounds fine. when fired with a 14 gr. there is a noticeable prolonged air discharge. When fired with a 21gr. the air discharge is much longer. Can the escaping air hold the valve open? If so why when at longer strokes? When I go to a shorter stroke with kodiaks down to 850fps. it works fine. Am I just too greedy lusting for unobtainable speed from my 18″ barrel? I am perplexed as to the cause of the air dumps, as it only dumps down to 1100psi. not all the way to empty.
Any ideas.
Thanking in advance…..

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A little more info. for you guys. I have an 18″ barrel, my valve stem and tophat are all in one (integral). I went to a heavier valve return spring, but my air supply is low, and I don’t have a pump so the heavier spring was only tested with 2200 psi. It worked fine so far but I guess it is back to the firehall for some air. With all of my air dumps, I’m their favorite customer. Oh, the barrel is not leaded, sparkly clean.
But it still works well on the crow in my yard today, took all the fun out of his day.
Thanks for the offers of ideas, keep them coming.
Once again, thanking in advance…

Fellow Canadian eh?! 🙂

The escaping air needs to be choked at teh valve stem to help close your valve.
Think of all the diameters exposed to HPA as pistons which are then pushing to keep the valve open. The easy fix is to add a suitable valve spring that works so that the net force is acting to close your valve.
Your valve sounds like mine. Did you see it somewhere or do great minds think alike? 🙂
Something else you can do is make the valve head larger beheind the seat so that it is a sail in teh wind of the escaping air.
Would sure like to hear more of your rig. Keeep up the good work.

You bet that pellet is holding the valve open until it can reach a peak pressure to either push the pellet out of the barrel or push the stem back into the valve. Have you tried putting in a stronger spring on the valve to force the valve closed faster?

It does really become a game of balance between the hammer spring and valve return spring to get an efficient system.

Sounds like too much back pressure on the valve with the heavier pellets and the longer barrel…

Perhaps the barrel is a bit leaded and offering too much resistance which builds up extra pressure against your valve, keeping it open and dumping pressure…

Just a wild ass’d guess.


Is the top hat grub screw tight?

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