By your picture, it appears that you are melting the filler rod onto the work-piece. In a word....WRONG!
You are supposed to heat up the work-piece, and when it's hot enough, dip the filler rod onto it, where it will melt to the work-piece. Not the other way around. The filler rod does not really go directly into the flame. If you do the opposite, you will get what you got. The process is quite similar to soldering, more than welding. Don't melt the part....but melt the filler rod into it.
If you can't get the part hot enough to melt the filler rod onto it, you may be using too small of a tip. Brass/bronze/copper will require a larger tip, as the part soaks up the heat and spreads quicker than steel & iron.
Not sure what effect acid has....but I have never used any. Get it clean, wire brush it, and braze, and clean as you go. You can see the crud on the surface as you heat, assuming you are wearing the right glasses.
Not sure what flux you used, but you may want to check that too.
Cast parts often have many impurities, including sand. When you heat them, the impurities will rise to the top, so many require brushing when you get started, and as you go. Use a stainless wire brush. If you don't have & keep things clean, the filler rod will not follow the heat well, and melt to it....like your pic as well.
Check your flame. An oxidizing flame will mess things up, and give you that popping.
You may be using too small of a tip. You need to get the part hot enough to melt the filler rod. If you don't and linger there too long, the part will oxidize, and you are again doomed.
You're way to high on the oxygen pressure. Should be closer to 5 PSI. Too much oxygen is bad.
Acetylene at 5 PSI is OK....3 would be better.
Don't use the cutting tip for brazing. Doable, but not easy, not for a rookie, and again, too much oxygen in the parts.
I've done it, but I've done a lot of brazing & welding. Never as controllable as a brazing/welding tip.
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