On Wed, 28 Aug 2019 19:36:11 -0700, pyotr filipivich
wrote:

Greetings

Been having trouble with circular saws. I have a Skill 5150, which
works well "for some values". Specifically, the saw blade will bind
and start to spin. Not good.

Got a replacement 5155 at Yon Local Pawn Shoppe, and while the
blade it came with gives no problem, when I swapped the rough cut
blade for the regular blade I had (the one which spun) - it too would
stick and spin. I was trying to cut masonite" - tempered hardboard
they call these days.
Two things I'd noted - label on this saw says "to prevent
kickback", tighten the retaining bolt a "1/8 turn past finger tight".
This is a new one to me.
Also, the blade I had is a thin blade, all steel with no carbide
teeth. the question I'm asking myself "Could it be that this blade is
'too thin' for this setup?" as in, the saw can't get a grip on it.

Other than that, it works fine, and I did manage to make the cuts
need for today.

Did you try putting a washer under the retaining bolt?