Steve,
You've provided precious little information to allow anybody to assist you. You haven't supplied the ADF make or model, or any other data, so at best anybody's response would be a best guess.
I'll throw in a little suggestion here in hopes that it helps. When you describe the antenna as being a whip type, my assumption is that it is a combined loop/sense antenna in the form of a blade antenna. If this is the case then you've essentially gone from mounting it right-side-up to mounting it upside-down. The 180degree error in pointing would be a natural expectation in these circumstances. Some ADF's had a pin on their connector which tells the box whether the antenna is mounted on the top or bottom of the aircraft. If yours has this you'll need to change its sense so as to match the location of your antenna.
