I have the FAST XFI system on my 94 Z28. I pulled the plugs to check them out. The front 4 plugs look great, but the back 4 plugs are really burning rich. Does anyone have ideas on why cylinders 1,2,3, & 4 are perfect, yet 5,6,7, & 8 are running rich????

 

If you're using the stock intake manifold its because of distribution problems at low rpm and idle. There is individual cylinder fuel trims in the stock pcm that vary about 12% iirc from front to rear. They kick off and all run the same at 14% throttle I think is the stock setting.