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Some Problems. :(

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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Some Problems. :(

Unread postAuthor: Phenom671 » Fri Jul 18, 2008 5:39 pm

Recently I made my first "real" spud gun. I've been experimenting with medicine bottles before and I really wanted to try this out.

Here's the problem:

Whenever I spray in my propellant (Aqua Net), screw on the cap, and hit the ignition system... Nothing happens. When it finally does go off, there's no "bang", maybe just a "whoof". The ammo I loaded doesn't even shoot out. I only had one day of firing and it finally died out. I cleaned the screws, I used for my ignition, of the hairspray gunk. Still didn't work. I swapped out the old screws and bought a new ignition system. Still nothing. Maybe the gunk from the Aqua Net in my combustion chamber has something to do with it?

I use ABS pipes, yes I know it's not recommended by most.
My barrel is 2" and 6 feet long. My combustion chamber is 5" and 8 1/2 inches long.

(nothing wrong with the spark, it's nice and blue. the screws are 1/4 to 1/2 inch apart at the most.

What do I do? Buy new pipes and start from scratch?

I'll try to get some pics up.
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Last edited by Phenom671 on Fri Jul 18, 2008 5:45 pm, edited 1 time in total.

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Unread postAuthor: frankrede » Fri Jul 18, 2008 5:41 pm

Your probably using too much fuel.
Try spraying just a little bit, less than a second.
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Unread postAuthor: MaxuS the 2nd » Fri Jul 18, 2008 5:42 pm

There's nothing wrong with your ignition system aslong as it's making a spark. It sounds like the fuel mixture is way off. Have you experimented with how much you spray in? Perhaps a fuel meter is in order?
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Unread postAuthor: Phenom671 » Fri Jul 18, 2008 5:47 pm

Yes I have. I read around here and that seemed to be the problem. I tried one short spray. Clicked it, and nothing. I've aired out the chamber and repeated with more or less fuel than before and still no combustion. I also tried blowing into the chamber to put in some oxygen and still no boom. Maybe I should try a more powerful fuel such as propane or butane?
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Unread postAuthor: starman » Fri Jul 18, 2008 6:00 pm

Get some pictures up and we'll take a look. 5" is an odd pipe size... are you sure it isn't 4 or 6".

You mentioned it going "whoof"...that sounds like there was no potato loaded. Also, are your spuds fitting in tightly and sealing the barrel? Fuel mixture can leak out quickly if not.

However, this is generally a fuel too rich problem. Air it out real good before you shoot...experiment with amount of Aquanet. Aquanet is usually pretty tolerant of this problem.
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Unread postAuthor: Phenom671 » Fri Jul 18, 2008 7:19 pm

I think it's a 4" . Maybe I got the extra inch from the threaded cap. Also it goes "whoof" because the projectile isn't being pushed out of the barrel. The combustion isn't strong enough to do that. And the ammo doesn't really seal it tightly. The reason why is, I don't have anything to sharpen the end of my barrel so it fits nicely and I use baby papayas. Yes I know, but potatoes are food and it would be a waste.
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Unread postAuthor: m4xr8d » Fri Jul 18, 2008 8:21 pm

Try a different fuel. Occasionally the mixes get off during production and what once worked now doesn't. Personally I've never had reliable/consistent results with Aqua-Net, I prefer Static-Guard. I've been playing with these launchers since the late '80s and have tried just about every type of aerosol spray.

A simple test:
With an empty cannon, hold your full aerosol a fist-width away from the chamber and spray a 2-3 second full pressure burst into the chamber. This should pull enough air into the chamber to ignite. Pull trigger. If it doesn't ignite, air it out and try another fuel.

Try not to cover your ignition contacts when spraying. If you do cover them, clean them off with some sandpaper.
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Unread postAuthor: Phenom671 » Sat Jul 19, 2008 3:00 pm

Alright, thanks. I'll try your suggestion.
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Unread postAuthor: SpudFarm » Sat Jul 19, 2008 3:07 pm

just bash the spud down there. no matter how bad you edges are it will cut a potato
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Unread postAuthor: psycix » Sun Jul 20, 2008 5:38 am

When the combustion goes whoof instead of bang, you have got way to much fuel.
Try adding about 1/3rd of that amount of fuel.
Also wait long enough to vent out the cannon after a shot (or a misfire) to let fresh air in.
Try to close down that spark gap. It should be less then 1/4" if possible.

For the ammo, use a potato that is larger in diameter then your barrel. Then cut out a cylindrical shape by pushing it into the barrel (a sharpened end helps)
You will have some leftover spud (a spud with a perfect circle shape hole trough it) that you will have to throw away, but since a big bag of spuds costs just a few bucks, this doesn't really worry me.
2" barrels ask for large spuds, not all spuds may be large enough.
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