My furnace manual says I can set the fan off delay on the PC board.my model has no such jumper that I can see.does the primary limit switch (open@140/close@100)control the blower? After gas shutoff the blower runs twice as long as I would like, sending cold air into the rooms.

If there is no set pin on your control board, then it can not be changed. Do you have a white: white rodgers board?

Exact model number of furnace?

Exact model number and brand of circuit board?