Then you are entering the wrong string in to FinderOuter. If it is showing up immediately, then it means FinderOuter is simply checking the string you have entered and telling you it has an incorrect checksum. It is not actually attempting to brute force any other possibilities.

Here's what you should be doing:

Open FinderOuter
Click on "Missing Base58"
Paste in your private key to the first box
Swap the invalid characters I and l for the missing char symbol (which is * by default)
Ensure the input type is "PrivateKey"
Click "Find" at the top

If it doesn't find your key, if you could then copy and paste here exactly what appears in the box at the bottom, that would be useful.