Word or idiom for: a sudden but short burst of enthusiasm?

Solution 1:

As usual, there is no one phrase that will fit all uses, but many that might be applicable.

flare-up is a very similar metaphor (both examples you gave engage in wordplay about fire), but its use is mostly limited to unrest and medical conditions.
flash in the pan also uses a metaphor of combustion, but applies mainly to brief effort or interest
As Greybeard suggests, nine days' wonder could be applicable for something that was briefly of general interest or celebrity
In many cases, a fairly literal translation might be the best route (including preserving the wordplay of the examples), like "brief [or feeble, spasmodic, short-lived, etc] flame"

Solution 2:

Flurry is another possible word for a brief moment of activity, meaning amongst other things "a brief period of commotion or excitement". There are also related meanings "a sudden occurrence of many things at once" and brief winds and snowfalls. (Merriam-Webster)

The original sense is from weather, and there are other more specific descriptions of the literal meaning, e.g. Cambridge's "a sudden light fall of snow, blown in different directions by the wind". This gives it the connotation of a burst of activity going in all directions, rather than a particularly organised and focused activity.

Lexico shows a lot of examples for "flurry of activity" and "flurry of excitement".

Solution 3:

"Nine day wonder / Nine days wonder / Nine day's wonder"

OED under the entry "nine"

3. In other allusive and proverbial uses.

a. nine days: the brief time for which a novelty is supposed to attract attention. In later use usually in nine days' (also day) wonder: an event or phenomenon that attracts enthusiastic interest for a short while, but is then ignored or forgotten.

1940 J. Colville Diary 8 Jan. in Fringes of Power (1985) 69 All this may, as Charles Peake thinks, be a nine-days' wonder.

1985 T. Lundberg Starting in Business vi. 70 There is great risk in becoming involved in a product that is a ‘nine-day wonder’ (e.g. skateboards).

Solution 4:

wave (n.)

Something that swells and dies away: such as

A surge of sensation or emotion

A wave of anger swept over her m-w

If you refer to a wave of a particular feeling, you mean that it increases quickly and becomes very intense, and then often decreases again. Collins

We combine, surge, swell and die away in waves of love, of lifetimes, so like the sea. Herb Haslam; Shadow Realities

“In the Future" portrays a brief wave of love and sympathy reaching the poet from a being on another planet. Joan Grossman; Valery Bryusov and the Riddle of Russian Decadence

I mean, think about it, isn't this the like of someone seemingly mesmerized by what we would call a short and temporary wave of love. Edeh Onche; In Love With the Girl-Child

Sloman dates the fourth wave of enthusiasm from Claimants' Union interest in the idea of Citizen's Basic Income during the early 1970s, and from a variety of individual writers taking an interest around the same time:... Malcolm Torry; A Modern Guide to Citizen's Basic Income