Semi auto piston valve concept help

[Major Collins]

OK from other threads on this site , i have heard you can create a semi auto with a piston valve ...correct?

this is done by using a spring behind the piston to reset it each time after being sucked back and actuated by a momentary valve such as a blow gun ...correct?

I have all these a piston valve fitted with a spring behind it and a blow gun. I i fill max to 160 psi with the chamber i got . What needs improving to get this to work?
Do i Just Need To crank up the psi or is it something im doing wrong ?

Please help me out

[Technician1002]

The Semi valve concept needs a couple of factors going for it to function. The valve has to be much faster than the chamber can vent out the barrel. The pilot valve must be able to re-close after the piston opens, but before the chamber pressure has vented. This is very difficult. Most semi piston setups simply use a smaller chamber that is rapidly replenished or use a piston valve with some kind of restriction between the piston and large chamber, so when the valve opens, the pressure at the valve drops rapidly so the valve re-closes. Often for a fast pilot valve, a pressure relief safety pop off valve is used. It is a real balancing act to get the pop off valve piloted piston to work. Many have tried. Only a few have been successful.

[clemsonguy1125]

Mine, correct, correct, Mine activates fine at 30 PSI, I assume your issue is it not sealing, make sure the piston is a good fit in the housing, try it by filing with a bike pump or single shot. Some pictures would be helpful.

[MrCrowley]

using a spring behind the piston to reset it each time after being sucked back

The piston isn't sucked back, it is pushed back. This may seem like I'm just being pedantic but if you understand the reasons why a piston valve works the way it does you can troubleshoot it (if it doesn't work), improve it or try and implement it in a different configuration (such as semi-auto). If you believe the piston works by being sucked back, you could make a bunch of pistons with no surface area differential (area of the sealing face which is exposed to pressure) and they would never actuate.

[Major Collins]

@ Mr.Crowly
"The piston isn't sucked back " OK my mistake it is pushed back by the air coming out of the chamber and pushing the piston back and going through the barrel.

@clemsonguy1125
Mine is a single shot i use a bike pump to fill to 160 psi and there is no leakage at the moment , i will fill in with picture when i get home today.

[clemsonguy1125]

Well if it works single shot, try hooking up a air compressor, then you can add a simple T-Mag with a detent

[Crna Legija]

you cant use the blow gun as is, you would have to take the leaver off and hit the button with a hammer. If your using a QEV then you need a hole in the piston to let air back into the pilot 1mm is plenty. If its home made it already has a EQ port. Thats if you want semi without constant flow of air

[Major Collins]

The Whole Setup (without chamber obviosly)


The 4 pistons i have - the one on the far left is the one i use (only that one and the brass one next to it actuate using the blowgun)


I didn't take a photo of the spring but it fits within the connector (the one the piston is housed in)well and tight.

[Major Collins]

so what suggestions should happen to my setup?

[jackssmirkingrevenge]

How is it "not working" exactly?

Does the piston not actuate?

Does it dump the whole chamber in a single shot?

[Major Collins]

well yes it dumps every thing in the chamber in a single shot and the piston resets after all the air is dumped.

how many shots should i be able to get with 250-200 psi?
and how strong would each shot be?

[Crna Legija]

Is you pilot valve just a standerd blow gun?

[Major Collins]

yes it is but i just sawed and grinded the bottom flat as you can see.
.... i use to have a ball valve but i changed it to a blow gun so its more trigger like

[jackssmirkingrevenge]

How big is your chamber? Are you tapping the valve momentarily, or giving it a great big squeeze?

[Major Collins]

I don't know the size in cubic cm's or any thing but its one of those burn o matic gas bottles.


this time i took out the spring and piston

And im not trying to squeeze the trigger the whole way but i might be every now and again.
i dont need to open the valve the whole way i presume?