Attached is a video that, if I uploaded it correctly, shows what happened when I hooked up a vacuum gauge to my A.I.R system to test my exhaust backpressure. I previously did the same test hooked up to the O2 sensor and the gauge did the same thing. The gauge only becomes more erratic as I increase the throttle. The car idles fine but shakes a little when doing so. The shake smooths out with RPM but the engine just will not go past 2000 RPM. It sputters and backfires around 2000 and I can't even make it up a decent hill! I was testing the back pressure because it seems like the exhaust is clogged somewhere in the cats or muffler. It looks to me now, though, that even if my exhaust is clogged that's not my only issue. I bought the car several months ago when it wouldn't start but there were just some fuses that weren't in all the way. Prior to figuring out it was the fuses there was damage to the plug wires (mice) and an MSD box hooked up so I replaced the distributor, ignition coil, wires, and plugs. I replaced the wires and plugs twice after I realized the shaking issue could be a misfire, but nothing has really changed. I'm not going to pretend I know what I'm doing and I know the most likely culprit are the areas I "repaired" myself, but I was very careful and slow replacing the plugs and wires the second time and I'm 99% sure they are all where they need to be. I set the timing after replacing everything and chased anywhere there could be vacuum leaks so I just have no idea where to go from here. it's a 92 Z/28 w/ the 5.7L 350. I'm happy to fill in any blanks I may have missed but I'd appreciate any help I can get!

 

I've removed more than one MSD ignition box for reasons like that.

Have you checked the valve timing?

 

Would the valve timing be different than the ignition timing? Probably a dumb question, but when I set the timing on my car I used a timing light to put it at 6 degrees past 0 but I haven’t done anything to the valves themselves. When I did set the timing the thing barely ran after I disconnected the est and really only seemed to mildly improve after I adjusted the distributor with my timing light. I’ve also never installed a MSD box so when I removed it I may have missed something when going back to stock-like components.