LEAN, HOT -No color and black spots, (oil) built up on porcelain. Cure: install a colder plug and richen fuellair mixture.
HOT -Absence of color on porcelain with dark shade on shell. Silvery-brown on ground strap indicates close mixture.
HOT, RICH-Black soot on shell and dark gray earth strap. Clean porcelain indicates too hot plug heat range. Colder plug and leaner fuellair mixture are required.
RICH-Good porcelain color with sooty-black shell. To correct, lean the fuel air mixture.
COLD, RICH-Porcelain and shell show heavy black deposits. Install warmer plug and lean fuellair mixture.
COLD-Black and sooty porcelain, shell shows normal dark color. Warmer heat range plug is required.
LEAN, COLD-Shell is clean to very slight coloring. Porcelain and ground strap are sooty-black. Correct with warmer plug and richer fuellair mixture.
HOW TO READ SPARKPLUGS
From Motorcyclist Magazine Dated sometime in the ‘70s
There's more witches brew and wives' tales associated with the art of reading sparking plugs than the number of tricks that can be found deep down in "The Magician's" famous black bag. On these pages we unravel all the mysteries, putting the spark plug in its proper place.
By Dave Ekins Photography/Pat Brallier
HOW many times have you had the spark plug out of your motorcycle? More times than you care to remember? Now, when you removed that plug was it simply to see if you were getting spark? Or maybe to determine if the motor was too wet to start? How about dry, no fuel at all? All this spark plug fiddling isn't necessary.
If you've got the correct spark plug you can leave it in there for a year and not even bother with it. If you're a racer then you're gonna pull the plug out every chance you get just to see if the engine's runnin' right, like all those other racers. It's all in knowing what's right. There are a few men who've made a profession out of reading spark plugs, but you can count 'em on the fingers of one hand. These guys are so scientific they can tell you if your points are dirty just by looking at a spark plug.
To begin with spark plugs come in heat ranges. There're also thread sizes, reach, and electrode style, but the part that concerns us is the number that refers to heat.
A B9E NGK for instance deciphers like this: B is for the 14mm thread, 9, the heat value and E is the 3/4" reach.
NGK's D8H for the Honda; 12mm thread D, 8 is the heat value, and H means a 1/2" reach.
The accompanying page gives us a picture of what a plug should look like when subjected to various conditions. It also tells us what these conditions are. Keeping it simple, the main reason one of us would want to make a spark plug reading would be to check for proper carburetor jetting, since we should be running the plug heat range recommended by the manufacturer.
If we've gone up in altitude the plug should give us a rich reading and we should change to a smaller main jet. (Changing needles, pilot jets, and slide cut-a-ways gets us into carburetor tun- ing and that's not our bag on this page.) Getting closer to sea level would require us to go in the other direction, richer, or a larger main jet.
We've also included a few pictures showing the wrong plugs in a given situation but this is just for a better understanding of what you might run into. It's all in the color and to make sure you've got the proper reading the engine must be run full throttle in the upper gears for a short time and then cut off cleanly, coasting to a stop. You should be looking at the plug with a 5x magnifying glass that lights up the subject; Champion among other people makes and sells 'em.
But before you load up that bike and take it out on your favorite deserted tuning road there're two things you had better look at first. Service the air filter, a dirty one won't do you a bit of good. Check the ignition timing, and while you're at it run an emery board or file across the points. You can get some horrendous spark plug readings when your ignition timing is way off and the air filter's acting like a choke .•
Spark plug in center is ideal condition. Porcelain can range from a light brown to dark chocolate, with the darker shade favoring 2-stroke-type engines.
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