Engine sputter after rain or wash

My '91 always sputters for a few days until it dries out after any rain or even being washed sometimes.

Any ideas what is getting wet and why it takes so long to dry out?

You might want to check if the cover for the coil and spark plugs is on the correct side. It should be RR for the rear and FR for the front(it's printed on the inside of the cover). Mines were reversed so the gasket was on the bottom. This let water in and it filled up inside the spark plug tube. Check out the rear side first. Pull the coil out and check to see if there is moisture on the stem. You might even see some rust coloring. If you do, don't pull out the spark plug as water may creep into the cylinder. Blow out with compressed air and let dry. Water had corroded the coils and even cracked the porcelin on the spark plugs. I bought new coils from Dali at almost half the dealer's price. SOS has those covers with gaskets on the top and bottom. I had no water problem with the front side. We get about 140 in. a year of rain here. Since the fix, I've experienced no such acceleration problems. Good luck!