Toyota Repair: drastic mpg changes, initial acceleration, jumper wire

Question
Hi Ted, I have a 1991 Corolla LE with a 4AFE DOHC
with 160,000 miles with a MPG problem. I have
been driving the car for the last 6000 miles and
the fuel mileage changes drasticly between each fill
up. It will be anywhere from 20.6, the lowest it has been
to 33.6. I have done a tune up, wires, plugs, cap & rotor,
pcv valve with no effect at all. The old plugs were bright
orange when I changed them but showed no fouling or
abnormal wear. The top end is a little noisy, sounds like a sewing machine and I am getting some valve clatter on
initial acceleration The check engine light has never come
on and otherwise is a fine running car. My next step is to
purchase a code checker and see what I find there. I was
just hoping you might could point me in the right direction.
Thanks for your time and Happy New Year!!!

Answer
If the check engine light is not on there will be no information stored in the computer, you do not need a code reader for this year model since it uses the OBD I diagnostic system, the codes can be retreived by placing a jumper wire between the terminal TE1+E1 in the diagnostic check connector under the hood, the only thing that I would be concerned about is the abnormal valve noise, was anything else done other than the tune up items.