Years ago I was working in North thailand and acetylene was out of stock. I made an adapter to connect to an LPG bottle and it worked well for cutting and brazing but when one tried to weld steel with it would pop and blow the torch out about the time the puddle began to form.
I assumed that temperature had reached some critical figure and the LPG was exploding and blowing the torch out. We stopped trying to weld and did repairs by brazing until we got a new supply of acetylene.
Never did know why and always wondered.
Bruce-in-Bangkok (Note:displayed e-mail address is a spam trap)