I've got a 80 SA that I've had for a few years now and have been trying to diagnose the starting issue.

After the previous starter went out I bought a brand new one that has recently developed a great problem. It will barely crank the engine at all when it previously would work perfectly. It spins just a little bit faster than if by hand. Barely even enough to register compression.

I've looked through the forums for anything similar to this but had no luck. I've tried using a newer battery, jumper cables, and still had no luck.

Just want to check if anyone has got ideas on what might be making it do this before I go ahead and buy a new starter or anything along those lines.

And I've checked all the fuses, all the proper rating and haven't blown

 

Have you replaced the battery cables and ensured you have at least one additional ground?

 

Ditto on the grounds. My RX7 only had ONE ground when I got it. I now have the grounds wired from Battery to Engine block, Block to strut tower, strut tower to alternator, and it runs SO much better and fixed a ton of electrical issues

 

Negative Battery ground to Alternator bolt will cure almost all charging issues. Get thick, stranded, quality cable and solder AND crimp your terminal connections, followed by a bit of grease which will wick down once it's heated up and prevent corrosion.

Also, be sure your ground to the starter is on the correct bolt, is clean, and better yet - replace it. 1980 was a LONG time ago, if that's an original cable. More grounds never hurt anything,