I bought a used skill 7.25 circular saw (#807) about a month ago for $10 bucks. I’ve cleaned it all up, checked the brushes and put a new cheap blade on it.  Everytime I use it however, the smoke alarm goes off.  It dosen’t appear to be binding.  It says 5800 RPM…maybe 15 years ago..now I would say it a lot slower…not that I counted.  Can someone suggest a cause of my problems?  thanks