REALLY high idle
 

I bought a 1987 GXL earlier this week, the previous owner put a fuel cell in it but didn't finish putting in the pump. when I installed it and started it it revs to 5-6k unless it's given just a hair of throttle, then it surges between 1 and 2k. I've checked all the vacuum lines and connectors I can see.

They do rev up when you start them. Probably best to post a video showing the symptoms

I know they rev up but they shouldn't rev this high
https://youtu.be/w_tcfp4mH0A

Ok, you've got a few options. First scenarios to check, you have a big honking vacuum leak, or your throttle might be held open. This can be reiterated by going through the troubleshooting page in the factory service manual, found here. RotaryHeads.com - 2nd Gen RX-7 PDF Technical Manuals FC3S

The section you will want is the fuel and emissions section. A few pages in, you will find a troubleshooting guide that will tell you to check the things I just told you, and has everything in order of operations. If you need advice, let us know and we will try to help. If you get through the manual and still haven't fixed it, we can try to find some other ways to diagnose. Also, if you do fix it, post the solution for others please.

You might check the codes on everything and see what it says. If you have cruise control (which can die and hold the throttle open), check the codes on it as well to make sure that system is functioning properly. I had to repair my cruise control computer and posted the rebuild on here if it helps.

Be safe and have fun

I found that the fuel pump pushes 95-120psi, could that be causing this issue? I've never messed with after market fuel pumps before .

Well now that you say that...

The page 4A-9 of the manual states fuel pressure should be approximately 35-37psi. It's shoving 3x the pressure through your fuel system. I would get that changed back to either a stock replacement, or something for a street car asap. Do you know if he upgraded the whole fuel system or just put in that tank and pump? If that pressure is running through the stock system, that's pretty dangerous. I know it's more work, but while you're getting fuel pressures figured out, please take stock of what changes have been done to the car.

The only thing he had done is put the fuel cell in, nothing else has been changed