Lots of things can effect your fuel mileage, so you need to do a little investigating to see what's going on.
The first thing to check is mixture. With modern unleaded fuels, oils, and plugs, reading the spark plugs is not nearly as accurate as it used to be. Don't waste your time trying to tune that way. Get an LM-1 and collect some real data.
What is your dynamic compression? If you havn't checked compression, then calculate your dynamic compression from one of the on-line sites, like K-B Pistons. If your compression is low, the engine won't be very efficient.
Take a look at your vacuum. It doesn't take much to temporarily run a vacuum line out the hood and around the windshield. Connect to a diagnostic vacuum gauge from Sears (or wherever). Low vacuum = high load = high fuel consumption.
Now take a look at your carb set up, especially the power valve and secondary springs. While cruising, I'd almost bet your vacuum is low enough to open the secondaries.
Don't forget to check fuel pressure and timing. If you have vacuum advance on your distributer, make sure it's adjusted properly.
Use a program like Desktop Dyno. If used properly, it's fairly accurate. Plug in all the right numbers, and take a look at the predicted torque curve. Your cruising rpm should be just to the right of where it starts to flatten out.
If the torque curve doesn't flatten out (relatively speaking), your cam doesn't match the rest of your engine bits.
And don't forget about normal stuff that effects every car - tire pressure,
oil viscosity, trunk load, etc. 2-3 psi of air pressure in the tires can sometimes make a big differance.
Obviously, big power engines in small lightweight cars can get decent fuel mileage. But you have to be careful about how you set the engine up.
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