I bought my 91 rs a couple of months ago. It ran pretty good and then one day it started backfiring and jerking pretty bad. It was pretty wierd. I put a tune up (plugs, plug wires, dis cap, rotor button, and fuel filter, oil change) and the check engine light was on. It told me to change the manifold air temp sensor so I did. It did not fix the problem. It does not always run bad, sometimes it runs great but mostly bad and also slow to crank when fired. It has a 350 tbi, with a 6 speed and msd ignition, msd distributor, and msd 6 box but the retard advance is not being used. Any ideas? Not sure what to do! Can anyone help?