fizzle vs fizzle out

fizzle is a noun but fizzle out is not a noun.

Word	Noun	Adjective	Verb	Adverb
fizzle	Yes	No	Yes	No
fizzle out	No	No	Yes	No

As verbs, fizzle and fizzle out are synonyms defined as:

Other synonyms of fizzle include peter out, taper off.

fizzle (noun)	fizzle out (noun)
a complete failure
a fricative sound (especially as an expression of disapproval)

fizzle (verb)	fizzle out (verb)
end weakly	end weakly