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spud gun 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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spud gun problems

Unread postAuthor: hyhatchi » Thu Jan 14, 2010 10:08 pm

Hello i am a brand new member and i need some help if possible. today i made my second combustion type potato cannon and for some reason it will not fire. so here are the specs its combustion chamber is a 2 foot section of 4 inch pvc that has a coupling to the 5 foot section of 2 inch so its pretty big! which i think may be part of the problem. i am using a grill ignitor for ignition and i put the two screws close to the 4x2 coupling so there near the front of the combustion chamber. which also may be why it doesn't work. then its 20 degrees where i'm at so i heard that hairspray condenses on the inside of the chamber so i left it in the house for about and hour before trying it while i fine tuned the screws and it gets a good spark but when i went out to test it it wouldn't fire. also i tried it with the end cap off so i could see if it would even shoot flame out the back but that didn't happen either. i think it may be the fuel if you have any suggestions for better fuel or whatever you feel is wrong with it let me know thanks.
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Unread postAuthor: twizi » Thu Jan 14, 2010 10:10 pm

you need a right propane air mixture go with metterd propane there is some articles in the how to section
here were you can buy a metirning dievice http://ultimatespudgun.com/fuel-metering-device-regulator-combo-p-178.html

there called spray and prays for a reason
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dont play with airsofter with 1000$ gun and play with a 5 dollar pair of glasses
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Unread postAuthor: hyhatchi » Thu Jan 14, 2010 10:40 pm

thank you for the suggestion but i would prefer a cheaper solution to propane as i am on a very very limited budget
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Unread postAuthor: Lockednloaded » Thu Jan 14, 2010 10:52 pm

you can't go much cheaper than propane (unmetered) I can get it here for about 4$ for a propane torch, and refills are cheap as well. Even with a meter propane shouldn't break the bank, a maximum of ten parts needed, and with galvanized steel, you can usually make one for about 30$
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Unread postAuthor: hyhatchi » Thu Jan 14, 2010 11:02 pm

so i could simply go to ace or lowes pick up one of those little blue propane torches and use it the same way i would hairspray?
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Unread postAuthor: coryS » Thu Jan 14, 2010 11:04 pm

You could be flooding it with to much fuel. Try 1 second than increase by 1 second each time you fill up. Be sure to air it out between each filling. Try using axe it doesn't condense or get sticky. And you can look into making a spark strip to make ignition easier
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Last edited by coryS on Thu Jan 14, 2010 11:08 pm, edited 1 time in total.
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Unread postAuthor: Lockednloaded » Thu Jan 14, 2010 11:05 pm

yes, but if you do not want to meter it out I recommend you also stop by a best buy and grab a PC fan for your chamber.
PS-Try to get one with out the ignitor attached, those are more costly, and you don't really need it
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Unread postAuthor: jimmy101 » Fri Jan 15, 2010 2:34 pm

If a squirt-n-screw doesn't fire, 90% of the time it is becuase of too much fuel. If you are sure the spark is working then the odds of an over fueling problem goes to pretty much 100%

For a very limited budget then go with either a butane lighter refill cartridge ($5 for zillions of shots) or a $1 cheapo butane lighter (~hundred shots). The lighter is pretty much the cheapest fuel source there is, even cheaper than Rightguard or Aquanet. Find a piece of insulated wire and pull the copper out to get a piece of tubing that will fit over the lighters outlet. Like this.

You can measure the flow rate of the butane if you want (bubble through water into an inverted measuring cup) or use a pure squirt-n-screw approach. The lighter supplies fuel much slower than do most aerosol cans so you probably need a couple seconds worth of fuel flow. The fuel flow from the lighter is very dependent on the temperature.

To get all the performance possible out of the gun you need to add two things. A meter and a chamber fan.

A meter can be done for a buck or two if you can find a store with 60ml plastic syringes. Farm supply stores often have them for giving shots to livestock. A syringe +needle should be perhaps $3 to $4 total. (If the gun is small enough you can also use the small syringes that come with printer cartridge refill kits.)

The fan can be really pricey if you go to BestBuy or RadioShack and get a computer fan. Much cheaper to scrounge one from somewhere (like an old computer) or try to find a "personal" battery operated fan at Kmart or Target. If the "personal" fan is small enough the whole thing can be permanently mounted inside the chamber. With a bit of luck you can get the fan for less than $5. You might also see if there is a computer recycler or repairer about. They will often give you a fan for a buck or two. A 9V battery and a switch is all you need with a computer fan. For the personal fan, it should have everything you need, maybe even the batteries.

IF you don't install a fan then you need to wait a while after fueling and before firing. Propane, butane etc take a while to diffuse and fully mix with the air in the chamber. Inverting a few times and waiting a couple minutes (particularly when it's cold out) will make the gun much more consistent.

Anytime you get a miss-fire you should fully purge the chamber with fresh air before refueling and trying again. With a meter and a fan the gun should never miss-fire unless you have a problem with your spark setup.
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Unread postAuthor: coryS » Fri Jan 15, 2010 2:54 pm

Keep in mind though with the personal fans though when you turn them on sometimes they spark because of the brushes on the inside of the fan, a normal computer fan is brushless so normally there is no spark. At least this has been my exp. I turned on my fan on one of my combustion cannons and it fired beause of the spark given off and scared the **** outa me.
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Unread postAuthor: hyhatchi » Fri Jan 15, 2010 4:09 pm

ok turns out it wasnt the fuel mixture but the ignition system. i used two screws that where coated with something that isnt conducting electricity very well so i am switching to machine screws and soldering the leads to the screws so i think i may have fixed it. thanks for all the input everyone
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Unread postAuthor: jimmy101 » Fri Jan 15, 2010 5:21 pm

coryS wrote:Keep in mind though with the personal fans though when you turn them on sometimes they spark because of the brushes on the inside of the fan, a normal computer fan is brushless so normally there is no spark. At least this has been my exp. I turned on my fan on one of my combustion cannons and it fired beause of the spark given off and scared the **** outa me.

First report I've ever seen of that happening. Way back when when fans first started to be used in spud guns people wondered if they could be used to ignite the gun. I don't think anyone (including me) could ever get it to work. That pretty much makes it the worst case possible. Doesn't work well enough to be useful, but does work well enough to get yourself hurt.

Hyhatchi: ok turns out it wasnt the fuel mixture but the ignition system. i used two screws that where coated with something that isnt conducting electricity very well so i am switching to machine screws and soldering the leads to the screws so i think i may have fixed it. thanks for all the input everyone

The best screws I've found are fine thread deck or drywall screws. They have very sharp points. You really shouldn't need to solder the wires. You can if you want, and it'll be a small bit stronger, but it really won't make the system work any better.

Heck, I use aligator clips to attach the spark source to the screws since I move the sparker from gun to gun.
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Unread postAuthor: artillerycreations » Fri Jan 15, 2010 7:04 pm

I would definately say that condensation on the walls of the chamber is happening. Like Jimmy said most of the time its too rich of a mixture. This just compounds condensation on the walls of the chamber. Once it has condensed it wont stop. You can try as much as you want but the only thing that worked for me was to wipe it out with a paper towel or let is sit for a half an hour. Propane meters can get to be around 30-50 dollars but for convenience they are worth it. In my area two 16.4oz tanks cost $6 and that much will last you the life of the gun. Spray gets expensive and messy quick. It also gums to igniters and creates resistance like crazy. Propane has more power, burns cleaner and stronger, can be metered, and wont mess up igniters.
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Unread postAuthor: starman » Fri Jan 15, 2010 10:06 pm

I have found spray and pray's, even decently designed ones to have less firing consistency in really cold and wet humid weather.

This is just my opinion but a spray and pray cannon should be nothing more than fun curiosity for demonstration purposes only. If you intend anything more than that, there's just no excuse not to use even a basic propane fueling system. It's too easy and cheap to do not to.
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Unread postAuthor: hyhatchi » Fri Jan 15, 2010 10:44 pm

ok i think i may have finally traced down the problem. the grill ignitor that im using had a 90 degree turn on one probe so i cut it off and removed the little plastic sheath over the wire because i assumed it would be like automotive wire and would still be insulated but i suppose im wrong because it seems like every time i try to work on it i wind up making the spark worse i changed the screws and spent a ton of time adjusting them and it would spark good for about ten clicks then nothing or erratic sparking and it seems every time the gap i use gets smaller and smaller so i think i may just go buy another ignitor and give it another go without cutting the probe off and trying to use the exposed wire like last time.
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