Hey all, I have a 85 rx7 GSL that I just picked up and need some help diagnosing an issue. At about quarter tank when driving I’m experiencing stuttering/jumping in the acceleration almost like a fuel cut. I have a new fuel pump installed and working but still have the issue. At full tank runs great. Any ideas ?

 

Rust pinholes in fuel pickup tube inside tank allowing air to be pulled into system.Could also be gunk in bottom of tank getting pulled into pickup tube. At this age likely the sock on end of tube has rotted off. Weight of full tank of fuel pins trash down and doesn't allow it to move around as much as when near empty. If you're certain the rest of fuel system is as it should be( I would pull fuel filter and tap it onto a paper towel to see what contents are.If there's rust/silt in filter,you're looking at tank issues) it's likely fuel tank needs some attention. Not an uncommon problem on 1st gens.
Is the new pump a Mazda fuel pump? Did you change fuel filter at time of pump install?