Combustion spud gun problems

tt22 · Sat Jun 24, 2006 7:03 am

i built a basic spud gun out of PVC Pipe... the combustion chamber is 24 inches long 4 iches in diameter with a sewer cap at the end... at the front it has a 4inch to 2 inch adapter and the barrel is 3 feet long.... drilled into the sewer cap is a fitting with threds coming from the inside of the explosion chamber to the outsid ewhere a ball valve is screwed on ... we use propane for a combustion gas... anyoen i am having problems launching taters we ram a tater(yukon gold potatoes) down to about the end of the barrel.... then we give the potatoe gun a charge of propane like 8 secs 10 secs 12 secs whatever... we have a simple BB-Q ignitor to create arc... so the first time we used it i gave it an 8 second charge of propane... and got no explosion my first shot then gave it another 8 second charge and that time got explosion.... the 2nd potatoe we loaded i gave it a 12 second charge and got no explosion then we gave it an 8 second charge and got nothing... we followed it up with a 10 another 8 another 10 and a last 8 no explosion.... we took off the sewer cap to see if we got arc and we do we just don't know whats the problem

does anyone know how 2 fix it?

Monkey621 · Thu Nov 30, 2006 6:23 pm

You need a more powerful spark, try a lantern lighter

pyrogeek · Thu Nov 30, 2006 6:46 pm

Is it airing out sufficiently between firings? Are your electrodes close enough, and clean? Is this a new ignitor, or one that was salvaged and not working well?

SpudStuff · Thu Nov 30, 2006 6:47 pm

You are using WAY to much propane. Add ~1 second of propane then shake the chamber. A small piece of plastic chain or similar object in the chamber will help.
You also need to vent the chamber after each shot to clear out the burned gasses which cannot sustain another shot. Vent it by removing the cap from the back then walking around with the chamber facing forards to blow air through.

SpudBlaster15 · Thu Nov 30, 2006 7:14 pm

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MikeNice · Wed Dec 06, 2006 10:57 pm

Agreed, fuel air mixture are off. Also, what are you using for an ignition? Dual spark from a BBQ ignitor is relatively cheap and easy to do. Your C:B ratio is off too. My very first chamber was 4"x24" with one ignition point with a 36"x2" barrel. Funny. Is your gun an over / under design or straight zooka? Mine was an over / under, but anyway, it never fired well and when it did it wasn't very good. I cut the chamber down to 18" and added a dual ignition and a longer barrel. I went through a bunch of barrels and settled on a 48" x2". It's not the exact recommended C:B ratio but these sizes work pretty well. For a 24" chamber I would use 60" to 66" barrel length to get some efficiency out of it. Good luck.