Getting white smoke backfire and singe marks in threads

[mick04u]

I built a 6’ potato gun (4” x 2’ combustion chamber and 2” x 4” barrel). I’m using aqua net hairspray for the propellant. When shooting, a puff of white smoke comes out the end cap that is screwed on hand tight. The igniter is screwed into the cap. There is also burn marks on some of the threads. Could spray have gotten on the threads? Could it not have been tightened enough??

[jimmy101]

That is likely just residue from the spray and that kind of cap isn't meant for pressure so it is leaking a bit (not a big deal).

I would suggest (1) switching to butane or propane and (2) metering your fuel.

For a chamber that big, you probably need something around 100 ml of gaseous fuels. You can get 60 ml syringes (that is about as big as you can get) at farm supplies stores and from Amazon. While you are at it, get some blunt tipped syringe needles.

You can use a disposable butane lighter or a propane torch as your fuel source.

Calculate the volume of your chamber then calculate the proper amount of fuel (propane and butane will need different volumes).

spud_wiki/index.php/Common_Fuels_for_Co ... n_Spudguns
spud_wiki/index.php?title=Fuel_meter
https://www.inpharmix.com/jps/Fuel%20Me ... ghter.html
https://www.inpharmix.com/jps/Hacking_lighter.html