All the above, plus check for any vacuum leaks. You may also want to replace the 02 sensor(s) if its never been done.
Unfortunately, there are
many engine factors that can affect fuel economy.
As far as removing carbon, you can use a plastic water misting bottle. Bring engine up to full operating temp. Remove intake piping from intake manifold. Set to high idle and squirt (mist) water into engine intake opening. A pint or so should do the trick.
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