Wood – Tooth Count for 6 1/2 inch Circular Saw Blades

I'd use the 24 tooth blade, but be sure it is a carbide type and sharp. Since all your cuts are end cuts and will be covered with baseboard trim, so getting an ultra smooth cut is not that important. Obviously, you don't want to see any large chips on the cut edge, so do your cuts slowly and smoothly. Save your 48 blade for visible finish cuts on softer wood. If you do plan to do any precise finish work, moldings etc, consider investing in an 80 tooth blade for that type of work. Good Luck.

Honestly, a 12 year old black and decker probably needs to go to the scrap yard. Low end saws are not built to be rebuildable, at least since the late 20th century (some 50 year old "low end" saws were a lot less "low end" in construction) - a 21st century low-end saw was usually built with "planned obsolescence" from the design phase forward.

Provide the exact model of your saw to check some details.

The one thing (if that's been left serviceable on this model) it might be worth trying is to replace the brushes if they are excessively worn. Beyond that, replacing the saw is far more likely to be cost-effective.