I have an 85 z 28. It was originally an LG4 car. Now it has a 74 355, 9.75 to 1 compression, cam with .440 lift. I am still running the computer controlled carb and distributor, but a bunch of the poulutin stuff is missing such as the air pump and injectors, the egr valve, and the cat isnt doing much. The pipe that comes off it into the engine bay is just plugged. The car runs ok but could probably run a lot better. I also only get about 10 MPG. I am just wondering how is computer reacting to the different engine and the lack of some of the polution gear? Is the computer all out of wack since it is a pretty different setup then it was designed to do. The car is also a 5 spd if that matters.

 

I am having the same problem. I have just started looking into it and it seems that the sensors all have to be operating correctly and hooked up to give feedback to the ecm. Without feedback, you are just running in open loop configuration where the computer doesn't know how much fuel to supply via the injectors. Hence the poor driveability and 10 mpg. I would start with the O2 sensor/cats and get those working properly then check the MAF. Also check if any sensors got missed or left out with the engine change. Mine is a 90 and is a Speed Density system so it is different than yours but is the same general principle.