How do I "read" my spark plugs?

[Unregistered Guest]

How do I "read" my spark plugs?

[Don Moyer]

There are many generic hand-outs showing various spark plug conditions and explanations on what the condition indicate in terms of what's going on within the engine. Because of the low compression design of the Atomic 4 and the fact that we normally use the engine in a relatively low power range, many of the common indicators are simply not readily apparent. Here are a few of the most meaningful observations you can make, specific to the Atomic 4:


Gray dry sooty look on all 4 plugs - Normal


Black dry sooty look on all 4 plugs - Usually indicates a slightly rich fuel mixture. In some cases, this condition could also indicate a slightly elevated exhaust backpressure (over 3 psi).


Black dry sooty look on only the fourth cylinder - In late model engines, this condition usually indicates that the fourth cylinder is being overcooled by too much cold raw water being directed to the rear of the engine by the diverter cap on the end of the "T" fitting.


One or two plugs with a black wet (gassy) sooty look and the other plugs normal - This condition indicates that something is going wrong in the two affected cylinders. Examples include:
Bad spark plugs
Bad plug wires or distributor cap
Low compression on the affected cylinders. If the two bad cylinders are next to each other, a partially failed head gasket may be indicated.


Black oily sooty look - This condition usually indicates worn or broken oil rings on the affected cylinder. If all four plugs are black, oily and sooty, it could indicate that exhaust backpressure is elevated to approximately 5 psi or higher.

[Bryanbt]

This is my first winter with the (new to me) 1971 late model Atomic 4
By all accounts the engine worked fine all summer and had not given its previous owner any major problems
During the winterising last week I noticed that the two aft plugs were wet and oily. The forward two were fine.
Is this the kind of problem I can live with for another season or more, or is it something that should be investigated further and fixed right away?

How can I diagnose it more specifically given the list of possibilities in Don'e post? (I am not even sure I fit under #4 or #5, though I think #4 fits the bill better):

Don'e post was:

"4. One or two plugs with a black wet (gassy) sooty look and the other plugs normal - This condition indicates that something is going wrong in the two affected cylinders. Examples include:
Bad spark plugs
Bad plug wires or distributor cap
Low compression on the affected cylinders. If the two bad cylinders are next to each other, a partially failed head gasket may be indicated.


5. Black oily sooty look - This condition usually indicates worn or broken oil rings on the affected cylinder. If all four plugs are black, oily and sooty, it could indicate that exhaust backpressure is elevated to approximately 5 psi or higher. "