Some black spark plugs
 

I have a 70 camaro with a 350. I have a new intake, 4 barrel carburetor (that I have jetted down to 58), and headmen headers. I have been having real bad gas mileage so I checked my spark plugs. I noticed that they range from being tan to black and everything in between. What would cause some to be black and some not?

Bad valve guides or bad rings allowing oil into the combustion chamber. 58 is low but I have seen 62 on a mild 350 with a 600 Holley. Do a compression check to test the ring seal. I would look for even numbers cylinder to cylinder instead of how much pressure its building.

Your driving a 70 Camaro what do you think good gas mileage would be? Keep your foot out of the carb that is the only way. Then its a trip to overdrive city if you want to knock down modest numbers.

Good gas mileage for a 350 with a 1-1 trans output, and something around 3.50-3.73 gears is probably still not gonna be more than 17mpg on the hiway at 60mph, IF you don't hit it hard. Around town most of us will drop 4-5 mpg with start and stop driving, and the lead foot problem.
I am getting nearly 17 mpg on the freeway with twin 450 Holleys on a tunnel ram at 6- mph, but around town I tend to be hard on the throttle. My plugs always show a bit dark when I check them, unless I just get off the freeway and check them.

Val's gotta show that thing off around town. SOB will pass everthing on the street except a gas station. God I want a ride.

They look more gas fouled then oil fouled but I will still do a compression test.

Check your timing. Can't imagine with the carb and smaller jets you'd be running rich with proper timing, unless you've maybe got a bad power valve.