That will be a Pinto engine I would think (OHC), it's worth giving it a tune up and checking the timing etc, by how you describe the plugs it's well deserved!

Ideally the plugs should be a chocolate brown colour indictating ideal fuelling, black sooty plugs are a sign of overfuelling but it's as well to see how it goes after a tune, then go from there.

But if you suspect you have a different carb, this will also cause problems, they could of fitted that from a 1600 model etc but on your smaller engine will cause it to run rich, if it's not been setup etc.

Martin