yellow spark plug reading

[plymouth71]

I have a fresh 440 that I am trying to tune. The last spark plug reading showed a yellowish tan. I have read that this my mean I need a colder plug like a j63y. I am currently running a champion rj12yc. Does anyone know any different?

[cudabob496]

I would think it means you are running slightly lean, and may need to increase jet sizes (if its a holley anyway). Plug heat range is intended for different compression ratios I think? The higher the compresion, the colder the plug I've been told, but I wouldn't bet my paycheck on it.

[plymouth71]

thanks for your answer. I think I my still be running a little lean. I don't know how I am running a avs with afb 62/52 rods and .115 jets. the car runs rich at idle. I tuned it with a vacuum gauge untill I gat a steady vacuum reading.(10") I would think that should be more than enough jetting.