My '90 XLT shorty will run great for the first 15 or so minutes but then there is a noticible decrease in performance. It has backfired a few times on deceleration also. What would cause this? Hopefully someone has a clue. Maybe fuel filter?

 

There is a solenoid valve between the PCV valve and the canister. It's a cylindrical thingy about 1" in diameter and a couple of inches long. Check that one out if you are backfiring during deceleration.

Good luck