The film begins with rednecks in a truck towing another redneck on a board down a slope. The towed man crashed into a barbed wire fence. A city-dweller with his wife on holiday is harassed by them. He defeats the ring leader with a karate blow. I forget the rest.
maybe “Straw Dogs”?
https://en.wikipedia.org/wiki/Straw_Dogs_(1971_film)
the board-and-fence scene is at the beginning of The Shuttered Room (1966). but the rest of your description does somewhat match up with Straw Dogs. i suspect that you are confusing the two films.
Thanks a lot. It was definitely The Shuttered Room, I now see. I only saw the beginning of the film at the time. The karate blows too are there in The Shuttered Room.
Solved!