Alison:

Another source of problems with amateurs who may have frequent backfires is in the nature of the Victor torch handle with its built-in check valves. What may be happening is that backfires are blowing carbon deposits down to the valve and check valve area where they are impeding gas flow and/or causing uneven operation of gas flow through sticking check valves. I've heard that the newer Victor torches with built-in flashback arresters are even more sensitive to "beginner abuse" because of the additional restriction of the arresters. In any case, I would remedy this situation quickly, since damage to equipment and possibly, your students, may result from a flashback situation.
Hope this helps!

your right you are never to use a wrench

A lot of good input....any popping or backfires you are getting is the result of the fuel gas pre-igniting before it reaches the end of the tip. This can be cause from several things, but ultimately it is caused either from a leak in the system, or an overheated tip (from reflected heat, or a starved tip).

Being a little more specific, here's a list of possible causes of popping and backfires: A loose tip, loose hose connections, damaged seats, damaged o-rings at the mixer, etc., will cause ignition of the fuel gas prematurely causing popping or a backfire.

Starving a tip is also a common problem. What I mean by this is, every tip requires a certain cubic feet per hour volume to burn properly. If this isn't met, the flame withdraws to find more fuel and it results in the tip becoming overheated...big time. An overheated tip will ignite the acetylene inside before it leaves the tip and it explodes inside the tip. Left unchecked, it can result in a flashback, which is very dangerous. This overheating from starvation can be caused from a variety of things: too small acetylene cylinder (or one which is near empty) will not provide sufficient volume for the tip....this is especially a problem with heating tips and larger cutting tips, however a near empty cylinder can cause this on ANY tip; not setting the line pressure at the regulator properly for the tip your using will also starve the tip. Not opening the torch valves sufficiently will also starve the tip. Note: When lighting the torch, open the acetylene torch valve, ignite the acetylene (by itself) and then proceed to open the acetylene valve until ALL the black smoke is absent from the yellow/orange acetylene flame BEFORE you open the oxygen valve. This will insure that you have sufficient fuel volume for that particular tip. If you cannot get rid of the smoke with the oxygen valve all the way open, you need to increase the oxygen line pressure at the regulator. If the yellow flame jumps off the tip BEFORE the smoke disappears, your tip is dirty! And this is another cause of backfires. Keep in mind even a new tip will require cleaning before use.....use tip cleaning files you can purchase at a vender. Yet another cause of tip starvation is restriction in the hoses, ie: too many splicers. An tip can also become overheated by reflected heat due to a tight space in which you are using the tip.

The other problem of flame fluctuation is likely caused from loose torch valves, damaged torch valve seats, or even excessive line pressure for the tip being used. Keep in mind for welding tips you need a 1:1 mixture of oxygen and acetylene, usually less than 5 psi each. For flame cutting, you will need no more than 5 psi acetylene pressure unless you are cutting heavier than 3 inch thick steel! Excessive line pressure in the fuel line will cause the flame to change continually because of the back pressure at the valve.

Be glad to field any other questions you may have!