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Recently we received a call from an MG owner who had just rebuilt his engine, but was unable to start it.
He had done all the appropriate tests, and was convinced he'd correctly set the cam and ignition timing. He pulled off the timing cover and rechecked the chain installation. He verified the firing order, and was getting spark at every wire.
The fuel system was working, gasoline was fresh, the carbs were clean, and the float bowls were filled properly. We suggested new spark plugs. "Too simple," he said, "something more serious is wrong."
"Try them anyway," we suggested.
Two days later he called. "Couldn't believe it! New plugs. Started with the first pull. Runs fine."
Later, another MG owner called to make an appointment for repair. He'd been driving in slow moving traffic on a very hot day when he suddenly lost power and began to run very rough. The car had been running hot, but had not boiled. He made it home, barely; the car would not go over 30. Burned some valves, he guessed.
By now you know what we suggested.
Next day, the call. "That was it. Really surprised."
About 15 years ago, after spending several hours "analyzing" why a car wouldn't run, and substituting nearly every fuel and ignition part on the car with no results, we replaced the two-day-old spark plugs. The car ran fine, of course, and based on that experience, we made a rule for our shop: any car with a running problem, unless explained by an obvious catastrophic problem (the con-rod thru the side of the block, etc.), first gets new spark plugs before any other tune-up procedure.
So why do the plugs stop working suddenly sometimes, for no apparent reason?
Usually it's a combination of a little bit of oil in the combustion chamber (some ring or valve guide wear, typical of most of our cars), plus a slightly rich mixture (our SU carbs get "richer" as they wear). It's aggravated by:
- High temperatures. The air is less dense, so the air/fuel mixture becomes still richer.
- High altitude, for the same reason.
- Stop-and-go driving, because the car doesn't get a chance to burn off some excess fuel and oil on the plugs.
Well, sometimes yes and sometimes no. The problem is contaminants often remain deep in the groove between the insulator and the plug body, and can be very difficult to remove.
