My shop is in an over-sized two-car garage attached to my home. My work is a calling, in the theological sense of the word. Ethical and moral questions arise every day – they are hard, sharp, unambiguous and unavoidable.

What Can a Spark Plug Tell You?

Posted: August 30th, 2013 | Author: Administrator | Filed under: Engine, Fuel & Air, Motorcycle Repair | No Comments »

Knowing how to read a spark plug can give you very helpful insight into what is actually happening inside of your engine. Below is a correspondence I had with a customer explaining what I could deduce from the state of his plugs.


I did a quick and dirty compression test on the cold engine – 140 psi on the left cyl and 150 on the right. These values will increase to around 160 psi when measured correctly on a hot engine – and that’s very good. So your engine is fundamentally sound and I can procede with the work.

Significantly, the two spark plugs looked very different and yeild clues as to the poor performance. The insulator tip can be read to give a good idea of what’s happening in the combustion chamber. The left plug was black with an oily sheen while the right was nearly white and dry. I suspect that one of the valve guide seals on the left cyl is allowing oil from the cams down the valve stem into the cyl and that the right cyl is running way too lean. I’ll take another plug reading after I have cleaned/rebuilt/adjusted the carbs. If the oil fouling on the left cyl is not too bad, it may be better/cheaper to just replace that plug once in a while rather than pull the head to install new seals. Plugs are cheap and easy to replace. Valve guide seals are not.


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