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wont fire..need help

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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wont fire..need help

Unread postAuthor: brettdotcom14 » Tue May 26, 2009 7:53 pm

i thought i was doing everything right.

18" chamber 4" diameter
bbq sparkier with spark strip 3 sparks
80mm fan
propane injection @ 30psi

it sparks it just doesnt shoot i dont know what the problem is

any help?
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Unread postAuthor: trigun » Tue May 26, 2009 7:55 pm

Is your chamber sealed otherwise propane may be leaking out to fast.

Also what size is your meter that is important to know.
- size of pipe
- length
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Unread postAuthor: boom_o_matic_2.0 » Tue May 26, 2009 10:44 pm

yea you might have too much fuel in it.
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Unread postAuthor: trigun » Tue May 26, 2009 11:09 pm

or he could have not enough we dont know because we dont know the size of his meter.
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Unread postAuthor: Technician1002 » Wed May 27, 2009 12:22 am

trigun wrote:or he could have not enough we dont know because we dont know the size of his meter.


A quick way to tell is to remove the cap and insert a lit match. too lean will provide no pop. Too rich will ignite as it goes into the rich mix through the dilute air interface. Keep your eybrows away. :shock:

From there the mix on the meter may be adjusted towards correct by changing the regulator pressure. Vent fully between tests.
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Unread postAuthor: qwerty » Wed May 27, 2009 4:08 am

Try adding a bit more fuel each time until you get a good mix
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Unread postAuthor: brettdotcom14 » Wed May 27, 2009 1:47 pm

i used the meter calculator so i thought itd be right

my chamber is sealed. the propane enters right in front of the fan which should spread it around good i thought. i do have a clean out on the end but thats screwed in tightly

its 3/4 pvc inner diameter is 7/8
7 3/4 long with the ball valve included.
according to the meter calculator it should be 29 psi
i used 20 psi before and that didnt help. tried 25 too. so i dont think less helps.

maybe ill try more, i just dont want the damn thing to actually blow up.

i had this thing shooting using axe when i didnt have the fan.

i was thinking maybe MAPP gass would be different not as finicky as propane?

thanks for the help everyone appreciate it.
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Unread postAuthor: jimmy101 » Wed May 27, 2009 2:38 pm

brettdotcom14 wrote:maybe ill try more, i just dont want the damn thing to actually blow up.

Important Combustion SpudGun Rule To Understand:
The maximum pressure occurs in a properly fueled combustion gun.
Too much fuel will reduce the maximum pressure, or cause the gun to not fire at all.

You are most at risk of a catastrophic gun failure when the gun is properly fueled.

Does the gauge read absolute (reads ~15 PSI normally) or gauge (reads zero PSI normally) pressure? I assume the gauge reads "gauge pressure" which is what the calculator probably assumes.

Your chamber is 18"L x 4"D with a volume of 226 cubic inch.
Your meter is 7.75"L x 7/8"D with a volume of 4.66 ci.
Propane in air is a 4.2% mixture if no air is displace by the fuel.
4.2% of 226 ci is 11.2ci.
So your meter to chamber ratio, taking into account the 4.2% mix, is 11.2/4.66=2.40
You will leave one volume of fuel at 1 ATM in the meter after the chamber is fueled.
So, you need 2.4+1.0 ATM in the meter. That's 50.0 PSIA, 35.3 PSIG.

Did you burp the meter? The first time you fill the meter it'll have a lot of air in it as well as propane. Fill, empty, repeat a couple times to purge the air out of the meter.
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Unread postAuthor: Technician1002 » Wed May 27, 2009 5:43 pm

jimmy101 wrote:
brettdotcom14 wrote:maybe ill try more, i just dont want the damn thing to actually blow up.

Important Combustion SpudGun Rule To Understand:
The maximum pressure occurs in a properly fueled combustion gun.
Too much fuel will reduce the maximum pressure, or cause the gun to not fire at all.

You are most at risk of a catastrophic gun failure when the gun is properly fueled.

Does the gauge read absolute (reads ~15 PSI normally) or gauge (reads zero PSI normally) pressure? I assume the gauge reads "gauge pressure" which is what the calculator probably assumes.

Your chamber is 18"L x 4"D with a volume of 226 cubic inch.
Your meter is 7.75"L x 7/8"D with a volume of 4.66 ci.
Propane in air is a 4.2% mixture if no air is displace by the fuel.
4.2% of 226 ci is 11.2ci.
So your meter to chamber ratio, taking into account the 4.2% mix, is 11.2/4.66=2.40
You will leave one volume of fuel at 1 ATM in the meter after the chamber is fueled.
So, you need 2.4+1.0 ATM in the meter. That's 50.0 PSIA, 35.3 PSIG.

Did you burp the meter? The first time you fill the meter it'll have a lot of air in it as well as propane. Fill, empty, repeat a couple times to purge the air out of the meter.


Often overlooked by some builders is the pipe size printed on the pipe is NOT the true ID of the pipe. That can cause rich mixtures as the meter will often have more volume than calculated. Lowering the regulator pressure a little to compensate may fix it.

Knowing which way to adjust is the very first thing to find out. Have you done the lit match test yet? Too lean will fail to light. Too rich will light.
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Unread postAuthor: brettdotcom14 » Wed May 27, 2009 7:27 pm

i didnt try the match thing yet. ill PM you i need alil more detail.

yes i measured the ID thats what is 7/8. says 3/4 on the pipe and i knew that wasnt right.

meter reads zero normal.

so youre saying 35 psi of propane? i dont know what PSIA is or PSIG. i assume psi air and psi gas?

no didnt know about the burping. ill try that also

thanks
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Unread postAuthor: brettdotcom14 » Wed May 27, 2009 10:10 pm

flipped my fan around and it shot at 35psi..weird ill have to try again tomorrow to see if it was just a fluke
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Unread postAuthor: jimmy101 » Thu May 28, 2009 12:13 pm

brettdotcom14 wrote: i dont know what PSIA is or PSIG. i assume psi air and psi gas?

PSIA = pounds/in<sup>2</sup> Absolute, the gauge reads 14.7 PSI sitting on your bench.
PSIG = pounds/in<sup>2</sup> Gauge, the gauge reads 0 PSI sitting on your bench. The gauge actually measures the difference in pressure between the outside of the gauge and the input to the gauge.

Most, but not all, pressure gauges are calibrated to read gauge pressure.


Technician is correct about the pipe IDs, I should have done the chamber calc with the correct ID for 4"D SCH 40 PVC, which is 3.998". :lol:
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Unread postAuthor: brettdotcom14 » Thu May 28, 2009 2:56 pm

oh ok thanks

well it was just a fluke darn!

anyway there was a problem with my fan switch so i took everything apart so i cant test much more.

but i guess its just the matter of getting the psi right
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