This is probably a Butthead question.

Backpacking liquid fuel stove manufacturers tell you that if your stove doesn't properly light after the priming, you're supposed to turn it off, wait a few minutes for it to cool, and start again. Why? What's the risk of just letting out a little more fuel right away and letting it prime some more? Our old green-suitcase Colemans prime with the fuel valve open, which surely is more dangerous than opening the fuel valve when the stove isn't fully primed?

The stove may be hot enough to ignite the fuel, resulting in maybe a big poof--think eyebrows, nylon, etc. Also, with the vaporized fuel, it only burns on top of the burner, rather than heating the fuel on its way to the top. If the stove doesn't start, at least wait until it stops hissing with an open valve. Once the fuel is liquid, it will flow to the bottom cup and properly heat the stove so efficient vaporization will take place.

Latter comment applies to stoves such as MSR/Svea which have a small cup at the bottom to collect fuel for preheating.

MReid is absolutely correct. If it does not light first time, let it cool down. Re-prime, it may not have been enough the first time to heat the stove.
Priming a warm/hot stove can result in a spectacular fireball.
Coleman's mix air and fuel from the pressurized tank so will light without prime, still many need to run some time to heat up enough for a good flame, again if they go out be careful re-igniting.

butthead

PS: If you don't mind, what stove in particular? bh

As the others have said yes the fuel could ignite due to the hot stove. Thus resulting in a fireball that if you are down close to the stove with you face may result in your eyebrows being removed. I’ve seen this happen a few times with people reigniting their water heaters.

butthead: "MReid is absolutely correct. If it does not light first time, let it cool down. Re-prime, it may not have been enough the first time to heat the stove.
Priming a warm/hot stove can result in a spectacular fireball.
Coleman's mix air and fuel from the pressurized tank so will light without prime, still many need to run some time to heat up enough for a good flame, again if they go out be careful re-igniting.


butthead

PS: If you don't mind, what stove in particular? bh"

MSR Whisperlite International.

TominMpls: MSR Whisperlite International. "

With that stove you have three concerns: potential fireball if the stove is hot enough to ignite the gas coming in; flare up (1-2 foot high flames) if it's a bit cooler and you light the gas before it is vaporized; and extended warm up period because the gas is burning on top of the burner rather than in the cup below. The cup holds gas to warm up the coiled fuel line above it, so that by the time the fuel hits the burner it's vaporized. If you're careful (and wait a bit), you can prime it while still warm (not hot)--opening the valve early allows the gas to enter the warm stove, usually causing a hiss as it is vaporizing some of the gas. Once the fuel cools down the stove, it will drip down to the priming cup. Once you have liquid fuel in the priming cup, it's ready to preheat again. You don't save much time doing it this way, and you still risk potential flare up. The flare up can be impressive!

Okay, that makes a lot of sense. You guys know so freaking much, thanks for sharing.

Whisperlite International lighting

I like to make sure valve opens easy, run a small bit of gas into the cup, this fills the pre-heat tube. In this video I squirt alocohol fuel into the cup for priming (burns much cleaner, no soot), but liquid gas woks fine. Give it enough to fill the cup. Use enough fuel to prime for a minute or more, a filled prime cup will do. Turn off the fuel valve, then light the fuel in the cup. Allow most of the prime to burn down, the burner will start to develop blue flames at the waffle burner ports, and you will hear a hiss. Carefully open the fuel valve till it runs well.

You can add fuel while priming by opening the valve, (in the video about 1min 50sec, blue flame at burner, opened valve a bit too early, shut down then opened again to a good burn), be careful it opens with a small fraction of a turn, you do NOT want a flamethrower! Close the valve again till burner starts up.

butthead

PS: The real fireball culprit is the Simmerlite with a ported burner head. That can and will hold fuel in the burner as well as the prime cup. It is one I like often, but it's not for a novice. One reason it was discontinued early. bh