I have a '78 F-150 with a 400 motor. When changing the spark plugs, all the spark plugs looked "normal" with no deposits with the exception of the plug for the #2 cylinder. The #2 spark plug contained a heavy white ash deposit that was fairly soft. There was no sign of this deposit on any of the other plugs. What might be causing the deposit?

 

Hmmm. Might have just stopped on that cylinder. Don't know why it would be white. Are you using an octane booster? Oxygenated fuel?

 

Beartracks, I am using premium fuel, octane 91 without an octane booster, and I don't think we have oxygenated fuel in our area. The ford manual shows a picture of the kind of deposit I have on the plug, but it attributes it to a lead deposit from the gasoline. After looking at the plug a little closer, the deposit is built up on oly one side of the plug. Somebody mentioned worn valve guides as a possibility. Thank You.