I figured I'd bump this back up instead of starting a new thread.
Last fall, I was getting 24-25mpg on a tank average and getting up to 29-30 on the highway. Now my gas mileage is down to a hair over 20mpg. I rebuilt my carb last fall but couldn't get a power valve at the time. I was still getting mid-high 20s after rebuilding it. Now it's dropped down, but nothing else seems to have changed. The performance and drivability haven't changed.
I just put a new fuel pump in last week with no noticeable effect on performance or mileage. I change fuel filters regularly, and it just got a fresh one. It also got a new PCV valve in the fall. I put in an entire brand new ignition system (distributor, plugs, wires, ignition module, internally regulated alternator) last summer and timed it, as well as rechecked timing recently. I'm also using ethanol treatment with every fill-up.
The charcoal canister was disconnected and the port on the carb plugged before I got the car, could that be causing problems by not allowing the carb to vent properly? I'm also pretty sure my cat is plugged and I'm going to rip it out soon. I've got some valve noise but it sounds exactly the same as when I was getting better mileage. Are there any other things I should look at?