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What is this newbie doing wrong?

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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What is this newbie doing wrong?

Unread postAuthor: Jim Beam » Thu Jul 22, 2010 8:13 pm

Hi all:

Sorry if this question has been posted before. I used the search function but didn't see what I needed.

I've been having great fun for the past year with my first spray-n-pray cannon, it's very easy to get the right fuel/air mix (16"L x 4" dia. combustion chamber, 44"L, 1.5" dia. barrel).

So, I thought it would be smart to build a pistol that could shoot all the leftovers from the big gun. It has a 2" dia. x 5"L chamber with a 22"L x 3/4"dia. barrel. I tried to scale it down from the big one.

But, it just won't fire! I have a nice ~1/16" spark between 2 screws, I'm injecting propane at 3% to 10 % (the explosive range), but nothing is happening.

Any suggestions?
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Unread postAuthor: Technician1002 » Thu Jul 22, 2010 8:18 pm

Your assumed fuel mix is most likely incorrect. After a fail to fire, unscrew the chamber and try to light the contents with a lighter. If you get a flame that burns for a sec, the mix was way too rich. The edge of the gas cloud burns and continues to burn as the flame mixes in air into the space containing the rich mix. This is most likely the problem.

If it is too lean, it won't do anything and just act like air to a lighter.

It is really easy to get way too much fuel in small chambers.
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Unread postAuthor: jackssmirkingrevenge » Fri Jul 23, 2010 1:04 am

Technician1002 wrote:It is really easy to get way too much fuel in small chambers.

Indeed, you should consider syringe metering:


Another thing, try widening the spark gap. 3/16 would work better in my experience.
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Unread postAuthor: jakethebeast » Fri Jul 23, 2010 3:20 am

i think you just but too much propane in there, try with little less :wink:
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