One black sooty plug

[Retirement Toy]

I tried to use a vacuum gauge to adjust the carb idle mixture and the gauge was bouncing. Decided to check the spark plugs. They all look really good except for #8 which is black and sooty. I have 200 miles on the engine. The engine is a rebuilt 5.0, GT40 Explorer heads (fresh valve job), E Cam, Holley 600 carb, HEI Mallory Ignition. I checked the distributor cap and didn't see any cracks. From here I am lost.

Can you tuning experts give me some direction as to what to check next.

Mark

[Gumball]

Is it wet or dry?

[Retirement Toy]

It's dry.

[stevem]

Bad Plug wire or plug? If it's dry I wouldn't think it's oil.

[Retirement Toy]

I went and bought a new plug, will try that first. How long should it take to see on the new plug. Also, will a bad plug cause the vacuum gauge to just around?

[cobraguy13]

Have you had the engine tuned on a four gas analyzer? If not you may have a float set too high causing an extreme rich mixture fouling the plug. I have never tuned a carb with a vacuum gauge I have always used a gas analyzer, the dead plug can cause the gauge to jump. Also check the plug wire to make sure it is not finding ground somewhere else other than the plug. Without seeing the vehicle this is the best advice I can give, you may want to check other plugs to see if they are black also.

[Retirement Toy]

I pulled all 8 plugs, only #8 is black n sooty. Will a rich system affect only one cylinder?

[cobraguy13]

Not normally most likely a weak plug or faulty plug wire from what you have described.

[Retirement Toy]

Great, thanks for the advice. I will replace plug and wire tomorrow.