help with acetylene torch

[crosby]

I am trying to learn how to use a torch, and seem to have a problem with mine. I read my Welder's book, which gives pretty explicit directions. They have me using 1 to 2 lbs of actylene, and 10 to 12 lbs of oxygen. I am cutting the straps off the driveshaft of my one ton, and all was going well until all of a sudden, it stopped being blue, and turned yellow, and started popping. Like I was out of oxygen or something... I shut everything off, started again (lit the torch) and as soon as I added oxygen, it shut itself off (the flame went out). Twice I tried to start it again and it popped and shut itself off when I added oxgen on the torch tip oxygen valve. Both the oxygen and acetylene have over 100lbs in each bottle. What could be happening? The torch is pretty old, I bought it for $75 with both tanks out of the Want Ad, a local buy/sell/swap book you can buy at gas stations and the supermarket. Now I am afraid something is wrong and it will blow up in my face. Thanks for any help you guys can provide!!