$2 garage sale oxy-acetylene cutting torch

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?
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Acet around 5 is OK , bump that O2 pressure to more like 20-25 and try again . The preheat flame should not change when you depress to oxy feed to start a cut . To set acet pressure open the torch valve all the way , increase pressure until the flame starts to pull away from the head then back off until it comes back . Should be no smoke/soot from it if properly adjusted . Oxy pressure is going to vary with the tip size and thickness cut , but 20-25 is a good place to start .
--
Snag
I got this info from somewhere on the net , probably either here or SEJW ,
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Since s.e.j.w hasn't helped I'll repost this here:
I bought an old Victor portable welding kit with a J28 torch, 2-W-J welding tip and Uniweld 17-15 rosebud, which apparently consume acetylene much too fast for the MC tank in the carrier. According to Victor only a size 000 tip is within spec, but which size tip do people really use for light welding like thermocouples and auto body rust repair?
http://victortechnologies.com/IM_Uploads/doclib_8051_DocLib_4680_Victor%20Acetylene%20Cutting%20Tip%20Chart%20 (0056-0411%20Rev%20B)_Jul2011.pdf
jsw
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This is the Harris gas welding and cutting manual: https://www.google.com/search?hl=en&source=hp&q=harris+cutting+torch+manual&gbv=2&oq=harris+cutting+torch+manual&gs_l=heirloom-hp.12...0.0.0.3860.0.0.0.0.0.0.0.0..0.0....0...1ac..34.heirloom-hp..0.0.0.a4TNmmcPg2I
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On Monday, April 14, 2014 3:02:24 PM UTC-5, Cydrome Leader wrote:

You probably have 6920 cutting tips (most common) on your torch. Make sure that the tip is clean (use a cutting tip cleaner). Regulator pressure depends upon the tip number that you're using (stamped on tip barrel). See: http://www.harrisproductsgroup.com/~/media/Files/PDF/Requested%20Resources/TIPSonCUTTING.ashx
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snipped-for-privacy@gmail.com wrote:

That looks about right. The cutting tip I have is apparently stamped "1", so the chart indicates 35-40 PSI for the oxygen.
I did finally unscrew it from the torch head and there are half shiny metal/half dark looking surfaces where the tip mates with the head. I'm going to assume this is no good and it needs to be cleaned or corrected somehow. Links to the photos are below:
the torch itself
http://www.panix.com/~presence/torch.JPG
the mating surfaces of the tip
http://www.panix.com/~presence/tip-seat.JPG
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On Tuesday, April 15, 2014 6:05:44 PM UTC-5, Cydrome Leader wrote:

Maybe try cleaning the mating surfaces with some fine steel wool. Check for daylight between the mating surfaces or mark the mating surfaces with a colored marker to see how they seal.
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On Wed, 16 Apr 2014 06:45:26 -0700 (PDT), snipped-for-privacy@gmail.com wrote:

Water based only. NOTHING containing any oil or flamible materials near oxygen.
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On Wednesday, April 16, 2014 8:23:05 PM UTC-5, Clare wrote:

No seals in danger of burning here. Any fuel content of markings from magic marker would probably be negligible, but I agree to keeping the mating surfaces clean.
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snipped-for-privacy@gmail.com wrote:

well, I went ahead and cleaned the surfaces. I ran the the back of the tip on fine sandpaper to romove the high spots and brushed the brass part of the torch clean. The sharpie test shows what seems to be a good enough seal now.
Setting the oxygen to just over 20psi did really help. I was able to slice up some steel plate about 3/4" thick. Teh torch did backfire (if that's what t the loud pop followed by the flame going out is) a few times, but only when shutting off the oxygen before turning the torch off. Does this indicate a problem may still be present?
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On Mon, 21 Apr 2014 15:08:54 +0000 (UTC), Cydrome Leader

If you turn the gas off first it is usual that the torch will pop. If you turn the oxygen off first you get a large orange, smoky flame :-)
--
Cheers,

John B.
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It's definetely popped and gone out when turning off the oxygen on the torch head. I can't figure out why this is happening though.
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I forgot to do the bucket of water test that was suggested. Will try that next.
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I'm using a Craftsman cutting torch bought in 1967. Never had the problem you describe. But I do usually run it at 5 Acet/15-20 Oxy.
FWIW, the regulators for this set died years ago, repair guys disclaimed being able to get parts. The roech handle, cutting and weldeing heads just go on and on.
--
Mike Spencer Nova Scotia, Canada

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I suspect this one is from 1983, and I'd be the second owner. I bought hoses for it as I didn't have any B sized ones, and I already had regulators.

They were doing railroad work by me so I asked the workers for some of those large plates the rails sit on. They seemed to have problems hearing me until I explained I want them to practive with a cutting torch. They they just started to hand them over the fence. It was sort of amusing.
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On 4/14/2014 1:02 PM, Cydrome Leader wrote:

Leave acet at 4 for life unless cutting more than 1" thick. Min 20 O2 unless using extra small tips. For what you have in it, take it in, and have it rebuilt, and be sure the rest of your life. An on fire fire spewing torch isn't a fun think. DAMHIKT.
Steve
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