The way you describe the performance falling flat and a change in the sound of the exhaust indicates a change ignition timing.
I don’t know how timing is controlled on these cars (msd box, distributor?) but check to ensure whatever device controls your timing is not bad from the wash job.
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coil pack to the distributor. I wrapped it in plastic and kept water away from that area. It’s running good now. Idk what I did when I removed the distributor, but it seemed to do the trick. The car feels good, even in the 163 degree summer heat we have here.