1770 - Lewis Walpole Library
American Notes and Queries, Vol. 3, 1889 - Wm. H. Garrison
referencing a 1776 handbill
Digging through flour with your mouth sounds just awful, and possibly smothering. I imagine there was a lot of coughing involved.
Wit, Character, Folklore and Customs of North Riding
of Yorkshire, 1898 - Rich Blakesborough
but that they were apparently performed in sacks tied around the neck. That seems dangerous, but other events included cudgel play so clearly safety wasn't a top priority.
The Sporting Magazine, Vol. 4, April 1799
Grinning contests are unknown today, but were apparently popular in the 18th C. According to Simon Dickie's book Cruelty and Laughter, opening the mouth or showing teeth was considered vulgar, and it seems a grinning contest was not a display of light-heartedness. The participants would hold a barrel ring up as a frame and try to make the most hideous faces possible. Joseph Addison wrote in the Spectator (Sept. 18, 1711) of a contestant, "He did his part so well, he is said to have made half a dozen women miscarry..." and also included the rhyme,
The frighfull'st grinner,
Be the winner.
Illustration from Harrison's British Classics, Vol. IV,
A 1786 reprint of the 1st through 4th volumes of the Spectator
If you want to read more on smock races, see this informative blog post: