Well, my gas mileage on the highway is a lot less than my city gas mileage I think, which is amazing considering I have 2.56's out back. I'm thinking I possibly have very rich secondaries. I've no real way to test this, so I'm thinking about possibly just buying some secondary metering rods/hangers and slowly trying to lean them out. Is it possible that if I did this I would 'over-lean' it and so the engine would run hotter when the secondaries were opening or make the overall mixture lean and I would get pinging on the highway?

I would look elsewhere. Your secondaries shouldn't be in use until you are on the floor with that gear. And then only above 3k rpm or so on up. Read your plugs to check the mixture. My guess would be your advance curve is somehow messed up or if you have HEI, the module is breaking down at cruise rpm. Your plug wires could also be breaking down under load.

IS the reason the secondaries aren't kicking in because even though the Q-jet is a mech. secondary, the air valves 'know' not to open?

Correct. The air valve is controlled by 2 things. The spring (this is the tension you feel when you push on the plates) and the choke pull off canister. The vaccuum canister on the side of the carb.
Ron

You shouldn't be anywhere near using the secondaries on the highway at cruise. Therefore, it's probably not your problem.

It would be very surprising if you highway fuel economy is worse than your city fuel economy. The first step would be for you to check what your fuel economy actually is in both situations. What are you getting for fuel economy anyway? You should be able to pull off 17 highway/14 city, depending of course on how radical your combo is.

Mike

Thanks everyone for helping out.

I haven't really been able to figure highway MPG/city MPG since I have to travel on both all the time, but right now I'm averaging about 12MPG. My combo is not radical at all, it's the stock 350 from '71, 76cc chambers with flat top pistons (run 87 with ease...could probably run lower if they had it), with a Crane Energizer 272H10, TH350, 2.56 gearing. I did buy a K&N air filter and after that filled up, and I seemed to have used a little less than 1/4 tank for 100 miles, which seems to be a few MPG better. Then again, I haven't travelled at highway speeds at all.

If the sec spring is set too loose it can absolutely destroy your gas mileage. Take a look and make sure the sec aren't partually opening too early.