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Help with QEV

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
  • Author

Help with QEV

Unread postAuthor: caelicola » Wed Jul 02, 2014 8:32 pm

I've built a few simple ball valve guns, and now my goal is to create a semi automatic 6mm gun. After looking around I decided a QEV and a slide valve with a main reservoir and a firing chamber was the best solution. I built my QEV from this picture:

http://images4.hiboox.com/images/3308/0 ... 6028f8.jpg

but from iron fittings instead of brass.

My issue is that the piston seals, but does not slide back when I vent the gun.
I just step down the T to a schrader for filling and venting right now, and I have tried 3/4" x 4" and x 8" firing chambers to no avail. All the air still vents out the schrader (as soon as I disconnect the air supply from my compressor the schrader vents). I've tried from 20 to 90 psi. The piston is a cylinder of thick rubber (3-4mm or so). There is very little space for air to flow past the piston to the firing chamber, so I don't understand why is isn't being pulled back when I vent it.
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