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Problem with Spark Plug

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: D_Hall » Tue Sep 11, 2012 9:03 pm

Another possibility: While your bulk fuel/air may be mixed sufficiently, you may have localized pockets - well, *A* localized pocket of gas wherein air and fuel are not sufficiently mixed. More to the point: That fitting may provide a small "sheltered" area where fuel and oxygen don't mix well. And since your plug is buried at the bottom of the fitting....
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Unread postAuthor: MrCrowley » Tue Sep 11, 2012 9:11 pm

D_Hall wrote:Another possibility: While your bulk fuel/air may be mixed sufficiently, you may have localized pockets - well, *A* localized pocket of gas wherein air and fuel are not sufficiently mixed. More to the point: That fitting may provide a small "sheltered" area where fuel and oxygen don't mix well. And since your plug is buried at the bottom of the fitting....
That's what worries me too. I can't think of a way that I could extent the spark plug electrodes further in to the chamber; well not a method that wont have the centre electrode being blown off after every shot (since there's not much surface area to fix something to the centre electrode).

Thanks for the tip about continuity, Jimmy. I'll check it this afternoon.
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Unread postAuthor: jimmy101 » Wed Sep 12, 2012 6:26 pm

If it is a mixing problem then patience should solve it. Fuel the gun and let it sit for an hour or so before firing. Given enough time it will fully mix even without a fan and even in recessed pockets.

Of course you can't have any slow leaks if your waiting for that long between pressurization and firing.
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Unread postAuthor: DYI » Thu Sep 13, 2012 9:00 pm

I was about to tell you of the headaches I've had from a similar setup, but D_Hall described it exactly. That's quite likely to be the source of your problem.
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Unread postAuthor: Lockednloaded » Thu Sep 13, 2012 9:15 pm

Could you just stick a length of small metal chain in the chamber and shake it around to mix your fuel? Granted your hybrid's fairly heavy, but it'd be better then waiting for the gasses to mix on their own
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Unread postAuthor: MrCrowley » Thu Sep 13, 2012 10:11 pm

Lockednloaded wrote:Could you just stick a length of small metal chain in the chamber and shake it around to mix your fuel? Granted your hybrid's fairly heavy, but it'd be better then waiting for the gasses to mix on their own
With the tennis ball barrel, no chance haha. The best I could do is use its own momentum to swing it back and forth but I doubt that would mix it too well.

I'll look around for a S/S hydraulic 1"x1/2" bushing as they are usually more shallow.

Otherwise, I might see if I can drill out the centre electrode partially or completely and then stick a longer one inside.

edit: Thanks for the tip, DYI.
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Unread postAuthor: striker » Fri Oct 05, 2012 4:51 am

just out of interest what thread is your spark plug? 10,12,14,16 or 18mm?
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Unread postAuthor: MrCrowley » Fri Oct 05, 2012 6:09 am

It fits loosely inside a 15mm fitting so I assume it would be 10mm.
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Unread postAuthor: striker » Fri Oct 05, 2012 7:06 am

I the only reason Ive asked is because Im going to run a long reach plug 26mm(1.04") to try and over come the same problem from happening in the first place, But you can only get them in certain size threads IIRC 12 and 14mm maybe 16mm
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Unread postAuthor: MrCrowley » Sun Oct 28, 2012 3:01 pm

Finally got around to doing something about the problem:
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That's a 1" x 1/2" bushing with the spark plug pushed up through the bottom and epoxied in place. There's a 1.5mm gap around the outside of the spark plug and the inside of the adapter and the hex part of the plug is larger in diameter than the 1/2" hole: pretty unlikely this thing could fail.

With this configuration, the spark plug electrodes sit just past halfway in to the chamber.
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Unread postAuthor: jrrdw » Sun Oct 28, 2012 3:11 pm

Do a continuity test between the ground and where you attach your high tension lead. You'd be surprised how many shorted spark plugs I have to replace working on lawnmowers.
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Unread postAuthor: MrCrowley » Sun Oct 28, 2012 4:50 pm

I probably should be doing that before I epoxy spark plugs in to fittings :D
I'll see if I can give it a go later this afternoon.
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Unread postAuthor: MrCrowley » Wed Oct 31, 2012 8:09 pm

Had a look online but was still left confused about testing continuity so I just did a compression test as before: fill chamber to 140psi, disconnect a wire from one of the spark plug terminals and hold it nearby to see if it sparks. Everything seemed to work fine, it had no problem jumping the 0.05mm spark plug gap that was under compression and the inch between the outside terminal and the wire.
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Unread postAuthor: jackssmirkingrevenge » Thu Nov 01, 2012 1:08 am

Oh fire it already :)
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Unread postAuthor: MrCrowley » Thu Nov 01, 2012 1:55 am

My exams are over in 7 days, hopefully I can take it up to my friend's bach soon after. I'm revamping the ignition system too so gotta get that done quick once the parts arrive.

My will to fire this thing at the usual testing location is almost non-existent. After seeing how the NZ police have handled the Dotcom saga and that potato cannon "meth lab" I mentioned a few weeks ago, I'm trying to keep a lower profile these days haha. Not to mention that time I went to the shooting spot when there were police cars using it as a staging area.

Sick of not being able to shoot at targets too. Waaaa! :(

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