have you ever learned to do hole sets and flat sets for fox? Can you use snares in your area? You need to know the rules and as mentioned have some kind of instruction by first hand with a seasoned trapper or from instructional DVD's or manuals on trapping. If they offer a trappers course in your state then I recommend that you take it There are post sets blind sets , dirt hole sets and flat sets which will all catch fox and coyote as well as bob cat and coons. You have to use bait or scents that will target the type of critter your after.
A dirt hole set is probably your easiest to learn. Digg a hole like a ground squirrel or ground hog would make and place some crushed up mice in it with a plug of dry grass over your bait then dig out for your trap just to one side or the other(left or right) approx and 6" out from the hole. The hole should be about 6-8" deep and it works better if you dig into the side of a bank or dry ditch so the fox has to come in from the side your trap is on always set where they are traveling and always have your trap below the hole and bed the trap so it is solid( will not move) and the jaws are packed underneath so they will keep the trap from rocking. Your chain should also be staked under the trap if at all possible. Set your pan tension to about 2lb and your ready for Mr. Fox.
Good Luck and may your traps be full of fur.
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