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Need help with first piston gun

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Need help with first piston gun

Unread postAuthor: andrew67 » Fri Oct 11, 2013 6:01 am

I Have 18mm diameter/500mm long with approx 15to 20mm of piston travel aswell as 2 shrader valves one for filling the chamber and one for release,this gun has never worked after many modifications I release air and the piston won't move but I've found if I remove the inner part of the valve and attach the pump and pressurize it then remove the pump,air releases as removed and it fires,is it possible to modify this again or should I just use a faster flowing valve,any suggestions on what valve I should use or modifications I should do would be more than helpfull as want this to work now that I've seen the sort of power it makes!
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Re: Need help with first piston gun

Unread postAuthor: jrrdw » Fri Oct 11, 2013 6:10 am

I'd like to see pictures of the fill and pilot valves both outside and inside of the pilot chamber if you can post them please. A question: Can you reverse the valves and have the same results?
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Re: Need help with first piston gun

Unread postAuthor: jackssmirkingrevenge » Fri Oct 11, 2013 6:26 am

Sounds like the piston isnt a very tight fit. needs to be if you're using a marginal pilot like a schrader.
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Re: Need help with first piston gun

Unread postAuthor: andrew67 » Fri Oct 11, 2013 7:00 pm

If I take the back of the chamber I am able to blow down the barrel to push the piston back it's firm any tighter and I wouldn't be able to do that,the piston is made from copper pipe with rubber bump stop on the barrel end and plastic at the back it's about 20mm long I'll mould an epoxy piston from a piece of the chamber or should I put an I ring on the piston?but wouldn't that make it to tight a fit?
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Re: Need help with first piston gun

Unread postAuthor: jackssmirkingrevenge » Fri Oct 11, 2013 7:04 pm

It sounds like your piston is too loose.

Epoxy casting is an easy way to get a perfect fit.

All the info you need here: how-to-database/topic8919.html
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Re: Need help with first piston gun

Unread postAuthor: andrew67 » Sat Oct 12, 2013 8:57 am

Thanks for your help,but have still gotten the same result after casting a new piston,replacing the release valve with a blow gun hoping the gun would release more air more quickly but no difference I tried an oversize piston then slowly reduced it while experimenting with it,I've drilled all holes in the blow gun aswell as modifying the guns valve to have more air flow but still it will only fire if I remove the pump while chamber is under pressure! Is 20mm of piston travel to much?seems the air doesn't release fast enough with the blow gun!
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Re: Need help with first piston gun

Unread postAuthor: andrew67 » Sat Oct 12, 2013 10:17 am

jrrdw wrote:I'd like to see pictures of the fill and pilot valves both outside and inside of the pilot chamber if you can post them please. A question: Can you reverse the valves and have the same results?
i'll post a pic if it works I'll post more still learning to upload as I have to shrink the file size but I think I have it sorted now! Thanks for your help!
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Re: Need help with first piston gun

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Re: Need help with first piston gun

Unread postAuthor: jrrdw » Sat Oct 12, 2013 12:56 pm

The piston only has to move back 1/4 of it's diameter. Did you try switching the valve places? Maybe 1 has a blockage???
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Re: Need help with first piston gun

Unread postAuthor: andrew67 » Sat Oct 12, 2013 1:37 pm

Ok,my piston will be moving to far then it's moving probably it's length,but yep I tried switching valves,they both flow as they should,I'll post better picks disassembled tomorrow.
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Re: Need help with first piston gun

Unread postAuthor: andrew67 » Sat Oct 12, 2013 5:48 pm

andrew67 wrote:
jrrdw wrote:I'd like to see pictures of the fill and pilot valves both outside and inside of the pilot chamber if you can post them please. A question: Can you reverse the valves and have the same results?
i'll post a pic if it works I'll post more still learning to upload as I have to shrink the file size but I think I have it sorted now! Thanks for your help!

Here's more pics
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Re: Need help with first piston gun

Unread postAuthor: Gun Freak » Sat Oct 12, 2013 9:37 pm

How did you attach the blowgun? It looks to me, based on the last picture, that the flow to the blowgun is being restricted which means the blowgun isn't operating at its full potential. Also you should definitely try decreasing the pilot volume by lessening the piston travel... Although I can see how that could be difficult given the design of the chamber/barrel connections.
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Re: Need help with first piston gun

Unread postAuthor: andrew67 » Sat Oct 12, 2013 9:49 pm

All good now!ive been slowly decreasing the volume,and reemed out the blow gun and it's valve as far as possible,I made a new slightly shorter piston but I'm almost certain the piston travel was the problem. I built it with all threaded connections for that purpose of being able to adjust it easily,I'm just starting to get together so it's more user friendly then I'll make it look good!
Thanks heaps for your help it's taken forever to get it working,much happier I now I haven't failed!
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Re: Need help with first piston gun

Unread postAuthor: jackssmirkingrevenge » Sun Oct 13, 2013 5:17 am

Glad you got it functioning :)
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