As you can probably imagine welding; either with a torch or an arc; is a skill that takes much skill and knowledge to do well.
That having been said I would like to give a summary of some of the basics of gas torch welding.
There are two broad classes of gas torches: Air-Fuel Oxy-Fuel (oxygen-fuel)
A propane torch is an example of an Air-Fuel torch. As the fuel gas moves through the torch body enough air is mixed with it to enable it to burn with a clean flame.
Oxy-Acetelyne and Oxy-Natural Gas are examples of Oxy-Fuel torches. In them pure oxygen is used instead of air. This is done to create a higher temperature flame.
Since air is composed of about 70% nitrogen and 30% oxygen and since all gasses involved in the flame are heated to a high temperature it is apparent that using part of the heat of the flame to heat up nitrogen (which contributes nothing to the production of heat) is wasteful. so while it is possible to achieve high temperatures with an air torch, you can do even better with an oxygen torch and there is no waste of fuel to heat nitrogen.
There are three adjustments that are used on a torch: fuel flow, oxygen flow and tip size. Fuel flow is obvious, and on a torch with only one adjustment knob, that knob will control fuel flow. If there are two knobs then the other one will control oxygen flow ( I say oxygen because even with a Fuel-Air torch it is the oxygen in the air that is important). Even on a torch with only one knob there may be a way to adjust the air flow. If you have ever examined the burner on a gas stove you have probably seen a movable air control plate for adjusting fuel-air ratio. The tip size is not usually thought of as an adjustment but using a range of tips allows you to adjust the torch over a much wider range than if you had only one size of tip.
Lighting a torch can be very easy or quite difficult. It depends on your procedure and the type of torch. First of all, be sure to use a spark lighter, not a butane lighter!! A butane lighter in proximity to a torch is an accident waiting to happen. That small container of butane packs enough explosive power to maim or kill if a spark should melt through it. You should certainly never have one in a shirt pocket while using a torch, that is entirely too close to your heart! A standard spark lighter such as usually comes with a propane torch can also be bought at most auto parts stores and all welding supply houses.
Turn on the fuel a little and hold the lighter so that fuel is trapped in the spark cup, this is especially important with a propane torch, which can be hard to light if it is cold. Operate the spark lighter and with even a small amount of luck you should have a flame.
Once you have your torch lit open the fuel and oxygen valves a little at a time alternately until you get the size flame you need. A word would be in order here to point out how you will know when the flame is adjusted properly. If you have too much fuel, then the flame will burn with a yellowish color due to oxygen starvation. Too much oxygen is harder to see but should be avoided if possible. Start with too much fuel, a yellowish flame then open the oxygen slowly. You will see a bluish cone start to form in the center of the flame. As you open the oxygen valve the cone will become smaller and more distinct. Just as its boundary becomes solidly defined, stop adjusting. This is the hottest flame and does not waste either gas or oxygen. Too much gas or oxygen cools the flame and wastes gasses. Too much oxygen can also oxidize your workpiece.
Finally, we come to torch manipulation. The handling of a torch can only be learned by experience. If you move the torch too slowly you risk melting holes in your workpiece. Too fast and your workpiece may never get hot enough. The object is to apply just enough heat to do the job. Is the flame big enough? Is it too close to the work? Am I holding the torch at the right angle? Etc., etc., etc., … .I can only suggest that you do not get in too big a hurry and that you practice on a piece of scrap material first if at all possible.
In the case of the chainsaw fuel tank, try to see how much heat it takes to melt some of the pieces you have gathered for filler metal. Try to get to the point where you can melt only the surface layer, even on a thin piece. Pot metal does not usually require anything in the way of flux, as the oxides that develop do not usually interfere with the welding.
By the way I probably ought to explain why it is necessary to use flux sometimes. I mentioned earlier that too much oxygen in your torch flame can oxidize your workpiece. Even at room temperature oxygen is reacting with everything around you. This is visible as the iron that slowly rusts and the plastic that becomes hard and brittle and the aluminum that goes from bright and shiny to dull with a whitish color. All of these effects are caused by oxygen combining with other substances at a slow rate. As the temperature goes up the oxidation happens at a faster and faster pace. If it happens fast enough we call it burning. Some things are very easy to oxidize such as fuel and other things are very resistant to oxidation such as gold or platinum.
Oxides in a weld zone can cause a weak joint or even prevent you from making a joint at all. To use a specific example let us use aluminum. Aluminum oxidizes to aluminum oxide which is very hard and melts at a much higher temperature than the original aluminum base metal. Because it melts at such a high temperature it does not bond very well to the aluminum metal and represents a weak spot in a joint. A flux will combine with an oxide and cause it to melt at a lower temperature thus combining with the rest of the melt zone and allowing a stronger weld.
A way to see the effects of fluxing is to use copper and solder. Find an old piece of copper such as an old water pipe, one that has a brown or green surface from oxidation. Now try to melt some unfluxed solder onto it and make it stick. Try to “tin” the surface. The corrosion won’t melt and combine with the solder. By the time you have got the oxides hot enough to melt the underlying copper base metal will be a pool of molten copper. As you can see the oxides have to go. You should remove the oxides mechanically. Use a file, steel wool, sandpaper, steel brush, etc. to remove them. If you try to solder, now things will start out just fine but you will soon have problems again as excess oxygen in the torch flame and atmospheric oxygen combine with the hot copper. If you now switch to a flux core solder you will see how the flux seems to eat through the oxide layer so that the solder can bond to the copper underneath. This is a good example of what flux does for welding, soldering and brazing.
Different metals have different chemical properties and thus require different fluxes. It used to be a matter of fact and faith that it was impossible to solder or weld aluminum with anything short of an atmosphere controlled process like heliarc or neutral atmosphere soldering and brazing. Now the chemists have come up with fluxes that will allow aluminium welding without shielding gasses.
I have gone on for quite a while now but I hope you were able to glean some tidbits of information that you didn’t have before. I am a firm believer in reading and trying when I want to learn a new skill and don’t have a teacher.