I have a 89 5.0 GT with a new 302 engine w/ 24# injectors, X303 cam. I have purchased the Granatelli 90mm mass air sensor and have followed the very basic directions received with the part for installing.  I can not get the car to run correctly with it on.  If I replace it with the original stock one the car will run, but not at the full performance level.  It's not using the extra fuel with the stock MAS.  What am I doing wrong?  Any help? 

Have you checked to make 100% sure the air meter is for 24# injectors? Sounds to me like it may not be correct for the injectors you have.

 

The first suggestion that I have, is to verifiy that you have a MAS that is calibrated for the 24lb injectors and is intended for your application. There is a difference in a Supercharged MAS and a natuarally aspirated MAS. I would also verify what kind of air filter is compatible for it. Some Mass Air Sensors will not work with a cold air kit and must have the filter connected directly to the meter. How big is your throttle body? What size is the filter you are using? I also recommend disconnecting your positive battery cable for 3-4 minutes and then reconnecting it and starting up the car anytime you make a change to the combination.