Howdy UserNameTaken,
(And, Happy New Year.)
I think you are right in that your spokeshave may have a gob of cast iron sitting on the flat surface where the blade is supposed to be riding on. That's entirely possible. (And, that would explain why somebody gave it away - to you.) However, it could be that you have a large burr on the blade itself.
You need to take apart you spokeshave and get a close look at that surface, under the blade, where the blade sits. You may have to get in there with a thin file and deburr the surface(s).
Here is a link to an article / guide for a spokeshave tune up. (You can also Google "Spokeshave tune up" to find other useful stuff.)
http://www.cianperez.com/Wood/WoodDocs/Wood_How_To/INDEX_How_To_pages/Smalser_on_SpokeshaveTuneUp.htmI am a big fan of the spokeshave. After I did a tune up and then build a one dollar honing jig, I have been able to easily take off beautiful, paper-thin ribbons of wood from the back of my bows that are up to an inch wide.
Good luck, stick with it,
Canoe