A shady business is a “fly-by-night.” Is a shady restaurant business a “fry-by-night”? A pun is:
 
Q: What kind of company is a 24-hour hamburger joint?
A: Fry-by-night.
 
The joke has been cited in print since at least 1978.
 
         
Wiktionary: fly-by-night
Noun
fly-by-night (plural fly-by-nights)
1. A creature which flies at night; a nocturnal flier or traveler.
2. One who departs or flees at night in order to avoid creditors, law enforcement etc. (often used attributively).
3. (idiomatic, derogatory) A person or business that appears and disappears rapidly, or gives an impression of transience.
4. (idiomatic, derogatory) A traveling businessman or tradesman.
         
Google Books
One Hundred and One Hamburger Jokes
By Phil Hirsch
New York, NY: Scholastic, Incorporated
1978
Pg. 60:
What kind of company is a 24-hour hamburger joint?
Fry-by-night!
 
14 December 1985, Financial Post (Toronto, ON), “Frits success no ‘fry-by-night’ wonder” by Jennifer Henderson, pg. 1:
THE MENU here at Frits is simple. The staple is homemade french fries, served with hot dogs, or as hot cats (wiener and fries in a bun) or in an indigenous Quebec combo called poutine - fries topped with gravy and melted cheese curd.
 
Twitter
Chef Alfredo‏
@Chef_Alfredo
Chicken Charlie is no fry-by-night operation http://sbne.ws/r/2GBF
6:44 PM - 2 Sep 2009
 
Twitter
San Singhania‏
@jokestoday
What kind of company is a 24 hours hamburger joint? Fry-by-night!  http://www.webfoolish.com
3:10 PM - 22 Nov 2009
 
Twitter
Daily Jokes‏
@webfoolish
What kind of company is a 24 hours hamburger joint? Fry-by-night!  http://www.webfoolish.com
3:11 PM - 22 Nov 2009
 
Twitter
mark_balstad24‏
@mark_balstad24
What kind of company is a 24 hours hamburger joint? Fry-by-night!
7:44 PM - 29 Dec 2009
   
Google Books
The Mammoth Book of Really Silly Jokes
Edited by Geoff Tibballs
London, UK: Constable & Robinson Ltd
2011
Pg. 112:
What kind of company is a twenty-four-hour hamburger joint?
Fry-by-night.
 
Twitter
Lucky Score‏
@LuckyScoreNet
What kind of company is a 24 hours hamburger
joint?
Fry-by-night!
#Joke from #BurgerJokes in #Jokes, #Humor and #Satire
7:11 PM - 27 Oct 2017