I finally dug out the $2 garage sale Craftsman (really Harris)
oxy-acetylene cutting torch. It came with a cutting head of unknown size
which I just cleaned out and #7 torch tip.

It passes the soapy leak test when pressurized, and the o-rings appear ok.

I've not used one of these before but have run into a problem.

I set the acetylene until the flame is not a smoky mess. Then I add oxygen
at the cutting attachment valve (It's wide open on the torch handle) then
I push the cutting lever and the flame gets really vicious. When let go of
the cutting lever, the thing goes BANG and the flame goes out.

What might be going on here? I've got a new set of torch side flashback
preventers to attach before I play with this more.

The welding forums go on about how this is backfire vs. backflash and that
the torch is leaking air or the O2 pressure is too low or something is
broken. It's all vague.

I had the O2 as about 10PSI at the regulator and the acetylene around 5.
Not sure if these are correct-ish.

Suggestions?