Piddling with Plugs

By Glenn Goodspeed (June, 1997)

I was inspecting the spark plugs in my '63 1800 the other day to see how my rebuilt engine was doing after 5,000 miles of service, and to try to determine whether the fuel mixture and the heat range of the plugs were about right yet.

At 2,000 miles, I pulled the plugs and found white deposits on the electrodes, a sign of an overly lean fuel mixture, but also a sign that the plugs are too hot for the engine. I used the double whammy approach: made the idle mixture richer and installed plugs two heat ranges cooler. Judging from the appearance of the plugs this time, I almost overdid it. The deposits on the electrodes were brown, which is good, but the deposits on the rim of each plug were black, which can indicate too rich a mixture. After I put the plugs back in, I made the idle mixture a little leaner.

You may be wondering what a heat range is on a spark plug. Plugs operate at lower or higher temperatures depending on the composition of the alloy steel in the plug body. The alloy is carefully designed to dissipate heat at a given rate. This lengthens the life of the spark plug, helps burn away deposits, and helps the fuel/air mixture burn at a controlled rate.

The ideal mixture, like the ideal spark plug, depends on the type of engine and any modifications that have been made to it. The symptoms of a too-lean mixture are backfiring on acceleration, lack of power and rough idle, as well as white deposits on the spark plugs. Too rich a mixture results in heavy black deposits on plugs, black exhaust smoke and poor gas mileage, not to mention flunking smog tests. Engines with more miles on them require hotter plugs, because they tend to suck more oil into the combustion chamber along with the gas and air.

You really should adjust the mixture first for best engine performance and economy before experimenting with different spark plug heat ranges. Once the mixture is right, then the object is to find plugs that produce light brown deposits on their electrodes.

On the P1800, it's smart to remove each spark plug after 3,000 miles of use, inspect the business end with a magnifying glass, clean it off with a wire brush and check the electrode gap before reinstalling. You can repeat this at 3,000-mile intervals until the electrodes fall off and cause serious damage, or you can replace them with new plugs after one or two cleanings.

Maybe they've updated the Haynes manual by now, but my edition lists outdated Bosch part numbers for the spark plugs. By using spark plug cross-reference charts, I've been able to come up with current equivalent numbers for Bosch and NGK plugs. See the spark plug table below for a synopsis.

You can purchase plugs by the part numbers at any competent auto supply. If they don't have them on the shelf, they should be able to order them for you. Don't let them talk you into buying their recommended plugs if you have already decided to try another type. Often auto supply clerks will try to sell you a plug with a different heat range so they don't have to backorder the kind you want.

I haven't bought plugs for a few years, because I found a great deal on them in the J. C. Whitney catalog. This was when my engine was starting to burn some oil, and I wanted the hottest plugs recommended for it. By a stroke of luck, Whitney was offering NGK BP5HS plugs for $.79 each. Apparently they didn't know they would fit a Volvo, and they were recommending them for motorcycles. I was amused when they called me to verify that I wanted twenty spark plugs for my motorcycle.

Spark Plugs for Volvo B-18, B-20, B-30 Engines

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