If your plugs look like this the air bleeds are too small on the idle circuit. Mid 70's jets are small enough for a 750 cfm carb. Looking at this from a tuning stand point, a good high output ign. system won't allow the plugs to show any color, in other words the plugs will stay clean all the time. So from tuning fuel cars we have to look at the color change at the ground strap and the spark plug threads. First the ground strap shows the timing, and correct timing of the engine will have the burn mark or pattern on the ground strap in the middle of the bend. Then the correct amount of heat or air/fuel efficentcy will be shown in the amount of heat transferred to the plug threads, the correct air/fuel mixture will have the top 2 threads dry or black or grey depending on what color the plug threads are to start with. You may see a few threads that look wet but don't worry as this just helps to show better how far the heat is traveling. When you achieve this then the engine will be at it's peak tune.