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Propane leaving barrel

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Propane leaving barrel

Unread postAuthor: ninjamonkey127 » Thu Oct 04, 2012 11:59 am

Hello, I am a new member to this site and I also built my first potato combustion cannon. I just got done building it today, but i can't get it to fire. The problem is that the propane is leaving the barrel, even when its loaded. I tried stuffing more down the barrel but that doesn't seem to help. It did however fire for the first test shot but after that it wouldn't fire at all. I have no clue what to do. Help would be very much appreciated! :)
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Unread postAuthor: jrrdw » Thu Oct 04, 2012 12:11 pm

What exactly do you mean by "leaving the barrel"? You can see the gas vapor floating out of the muzzle? Air out your chamber very well, unload it and look through the chamber and force clean air through it. Next check your spark. Can you see it? If so cap the chamber back off and try a dry fire. If it still doesn't work check your mixture and be sure to clear your chamber in between each test.

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Unread postAuthor: wyz2285 » Thu Oct 04, 2012 12:42 pm

I think the problem it's that the meter pipe it's to far from chamber and most of the fuel are trapped inside that long house that connects the chamber/meter pipe, not well mixed inside the chamber.
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Unread postAuthor: ninjamonkey127 » Thu Oct 04, 2012 12:57 pm

What i mean is that i can hear the propane leaving the barrel. I will try to do what you suggested, jrrdw.
Noob question. How do you know if you have the right mixture?
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Unread postAuthor: jrrdw » Thu Oct 04, 2012 1:39 pm

ninjamonkey127 wrote:How do you know if you have the right mixture?


Here is a good page to start you off in you quest for info: http://www.spudfiles.com/forums/useful-links-and-infromation-t13096.html

To help you with the mixture: http://www.spudfiles.com/forums/propane-injection-math-t8017.html.

It is a good idea to shorten your feed hose to your meter...

Post some close up pictures of your cannon and meter so we can better help you.
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Unread postAuthor: jsefcik » Thu Oct 04, 2012 4:25 pm

better air mixure and maybe installing a chamber fan will help out alot!
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Unread postAuthor: ramses » Thu Oct 04, 2012 9:40 pm

You may hear the echoing sound of fuel leaking into the chamber through the hose out the barrel instead. None the less, if you have a spark, you can try the following qualitative test:

To tell if your running lean or rich, drop a match down the muzzle. If that lights it, you're running rich or with a poorly mixed mixture.

Otherwise, you're running lean.

Leave a decent amount of time between trials so as to minimize heat buildup. Obviously vent between every attempt.

Needless to say, point the canon in a safe direction (away from your head) when dropping the match and do not load a projectile.

If you're actually loosing propane out the barrel, having a projectile loaded will help (and prevent the use of this test.
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Unread postAuthor: jackssmirkingrevenge » Fri Oct 05, 2012 12:00 am

Love the configuration, good luck with getting it firing!

I say "luck", I mean of course "science".

Some details of your fuelling procedure and chamber/meter dimensions and fill pressure would help. There is definitely too much pipe between the second ball valve and the chamber.

I would invert the bottle and use the metal pipe attached to the chamber as the meter.
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Unread postAuthor: ninjamonkey127 » Fri Oct 05, 2012 6:32 pm

jrrdw wrote:
ninjamonkey127 wrote:How do you know if you have the right mixture?


Here is a good page to start you off in you quest for info: http://www.spudfiles.com/forums/useful-links-and-infromation-t13096.html

To help you with the mixture: http://www.spudfiles.com/forums/propane-injection-math-t8017.html.

It is a good idea to shorten your feed hose to your meter...

Post some close up pictures of your cannon and meter so we can better help you.

Thanks for the great info! I'm going to try to figure this out tonight. I'll post some pictures up later.


jsefcik wrote:installing a chamber fan will help out alot!

I actually do have a fan in the chamber, i just need to wire it up.


ramses wrote:You may hear the echoing sound of fuel leaking into the chamber through the hose out the barrel instead.

I never thought of that. I only assumed it was leaking out of the barrel is because it wouldn't fire.


jackssmirkingrevenge wrote:Love the configuration, good luck with getting it firing!

I say "luck", I mean of course "science".

Some details of your fuelling procedure and chamber/meter dimensions and fill pressure would help. There is definitely too much pipe between the second ball valve and the chamber.

I would invert the bottle and use the metal pipe attached to the chamber as the meter.

Thanks! Here are the dimensions:
Chamber 3"x 15 9/16"
Meter 1/2"x 6"

I might of over done it with the pipe but i don't plan on taking it apart and working on it any time soon. That first test shot has kept my hopes high.
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Unread postAuthor: jsefcik » Fri Oct 05, 2012 7:43 pm

ninjamonkey127 wrote:
jrrdw wrote:
ninjamonkey127 wrote:How do you know if you have the right mixture?


Here is a good page to start you off in you quest for info: http://www.spudfiles.com/forums/useful-links-and-infromation-t13096.html

To help you with the mixture: http://www.spudfiles.com/forums/propane-injection-math-t8017.html.

It is a good idea to shorten your feed hose to your meter...

Post some close up pictures of your cannon and meter so we can better help you.

Thanks for the great info! I'm going to try to figure this out tonight. I'll post some pictures up later.


jsefcik wrote:installing a chamber fan will help out alot!

I actually do have a fan in the chamber, i just need to wire it up.


ramses wrote:You may hear the echoing sound of fuel leaking into the chamber through the hose out the barrel instead.

I never thought of that. I only assumed it was leaking out of the barrel is because it wouldn't fire.


jackssmirkingrevenge wrote:Love the configuration, good luck with getting it firing!

I say "luck", I mean of course "science".

Some details of your fuelling procedure and chamber/meter dimensions and fill pressure would help. There is definitely too much pipe between the second ball valve and the chamber.

I would invert the bottle and use the metal pipe attached to the chamber as the meter.

Thanks! Here are the dimensions:
Chamber 3"x 15 9/16"
Meter 1/2"x 6"

I might of over done it with the pipe but i don't plan on taking it apart and working on it any time soon. That first test shot has kept my hopes high.



for wiring up the fan, if you wanna go cheap. use an old flash light works perfect , i used one for mine, i cut the spot on the flashlight were a normal button goes and put in my own, and just hooked it up,unless your going little bit more pricy with a full control switch
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Unread postAuthor: ninjamonkey127 » Fri Oct 05, 2012 7:58 pm

jsefcik wrote:
ninjamonkey127 wrote:
jrrdw wrote:
ninjamonkey127 wrote:How do you know if you have the right mixture?


Here is a good page to start you off in you quest for info: http://www.spudfiles.com/forums/useful-links-and-infromation-t13096.html

To help you with the mixture: http://www.spudfiles.com/forums/propane-injection-math-t8017.html.

It is a good idea to shorten your feed hose to your meter...

Post some close up pictures of your cannon and meter so we can better help you.

Thanks for the great info! I'm going to try to figure this out tonight. I'll post some pictures up later.


jsefcik wrote:installing a chamber fan will help out alot!

I actually do have a fan in the chamber, i just need to wire it up.


ramses wrote:You may hear the echoing sound of fuel leaking into the chamber through the hose out the barrel instead.

I never thought of that. I only assumed it was leaking out of the barrel is because it wouldn't fire.


jackssmirkingrevenge wrote:Love the configuration, good luck with getting it firing!

I say "luck", I mean of course "science".

Some details of your fuelling procedure and chamber/meter dimensions and fill pressure would help. There is definitely too much pipe between the second ball valve and the chamber.

I would invert the bottle and use the metal pipe attached to the chamber as the meter.

Thanks! Here are the dimensions:
Chamber 3"x 15 9/16"
Meter 1/2"x 6"

I might of over done it with the pipe but i don't plan on taking it apart and working on it any time soon. That first test shot has kept my hopes high.



for wiring up the fan, if you wanna go cheap. use an old flash light works perfect , i used one for mine, i cut the spot on the flashlight were a normal button goes and put in my own, and just hooked it up,unless your going little bit more pricy with a full control switch

I bought a project box from radio shack and had a switch and some wire laying around that i scavenged from electronic devices a long time ago.
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Unread postAuthor: ninjamonkey127 » Sat Oct 06, 2012 7:42 pm

So i test fired my gun again, but this time i put 30 psi of propane in the chamber instead of 40 and i had the fan wired up and on. It was a whole lot more powerful then the first time it shot and it had a lot of recoil to it. When i tried firing the gun again it wouldn't work. I was wondering, does it matter what kind of potatoes i use? I should also mention that a potatoe was left in the barrel over night. Maybe that has something to do with the gun firing?
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Unread postAuthor: jsefcik » Sat Oct 06, 2012 8:00 pm

ninjamonkey127 wrote:So i test fired my gun again, but this time i put 30 psi of propane in the chamber instead of 40 and i had the fan wired up and on. It was a whole lot more powerful then the first time it shot and it had a lot of recoil to it. When i tried firing the gun again it wouldn't work. I was wondering, does it matter what kind of potatoes i use? I should also mention that a potatoe was left in the barrel over night. Maybe that has something to do with the gun firing?


brand doesnt mean anything, and id say fresh is always best, would you use a rotten potato?

and you want a very tight seal with the potato, i wouldnt leave the fan on, just put it on for a couple seconds,
what kind of ingition are you using as well?
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Unread postAuthor: ninjamonkey127 » Sat Oct 06, 2012 10:33 pm

jsefcik wrote:
ninjamonkey127 wrote:So i test fired my gun again, but this time i put 30 psi of propane in the chamber instead of 40 and i had the fan wired up and on. It was a whole lot more powerful then the first time it shot and it had a lot of recoil to it. When i tried firing the gun again it wouldn't work. I was wondering, does it matter what kind of potatoes i use? I should also mention that a potatoe was left in the barrel over night. Maybe that has something to do with the gun firing?


brand doesnt mean anything, and id say fresh is always best, would you use a rotten potato?

and you want a very tight seal with the potato, i wouldnt leave the fan on, just put it on for a couple seconds,
what kind of ingition are you using as well?

Is there anything i can wrap the potato in to give it a tighter seal? cause every time i load a potato in the barrel it slides down really easily. and yes, the potato is bigger then the barrel bore.

I'm just using a bbq igniter.
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Unread postAuthor: MrCrowley » Sat Oct 06, 2012 10:51 pm

jsefcik wrote:i wouldnt leave the fan on, just put it on for a couple seconds
...and you continue to show just how much you don't know. Leave the fan on, I believe it increases turbulence and results in a faster combustion. Someone more knowledgeable on the subject can correct me if my justification is wrong but it seems to be pretty common practice nowadays to leave the fan on during combustion.
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