Do a heat test;
Conduct a heat range test as follows:
Drive around for at least 30 minutes to allow engine heat to build and then drive out someplace where you can run the car at a sustained speed on a straight road for a good distance of say a mile or two without having to slow down or stop, very important.
Run the car up to the maximum speed in high gear that you would typically drive if you decided to drive fast and I don't mean dangerously high speed.
Hold the car at that sustained speed and without letting off the throttle, reach over and turn off the ignition. Let off the throttle, push in the clutch quickly and coast over to the side of the road.
Open the hood and allow the engine to cool off a bit. Remove the spark plugs and examine the color and condition of the electrodes and insulators. The color will be a true indication of how hot or cold the plugs are running. But remember ... during the test if you let the engine slow down or let it idle and then check the plugs you will have changed the effects and not get a true indication. I've seen many a person let an engine idle, check the plugs and then claim they were just right only to discover later that the plug heat range was too hot.
It they are white, the plug is too hot. Go to a colder plug.
If they are black, they are too cold. Go to a hotter plug.
If they are a shade of tan, they are just about right.
Also understand that all this is not set in stone. A person who putts around at say 35 or 40 MPH and never faster may need a hotter plug. The lower the combustion chamber temperature the hotter theplug should be. On the other hand, as conditions cause the combustion chamber temperature to rise, a colder plug may be necessary. You just don't want the plugs to be sooty black and foul out which means they're too cold or chalky white which means they're too hot. You want the plugs to be somewhere in between like light brown to grayish tan. However, slightly white may not be too bad unless it is "chalky white."
Keep in mind that the plug descriptions are for engines in good condition, correct carb mixture, good hot ignition, etc., etc.. One with bad rings for example can cause oil to get on a correct heat rangeplug and foul it out (short to ground down the side of the center electrode) where it won't fire. It will be black and oily.
On the other hand, a plug that is running too hot will turn chalky white or bright white. This can reduce the life of the plug or even cause the electrode to burn up.
A plug can also get hot enough to glow which can ignite the mixture before the spark occurs (pre-ignition). This can be bad news... During pre-ignition the temperature inside the combustion chambers can reach WELDING temperatures in just a few seconds. The result is a hole can melt right in the middle of an aluminum piston! ... and to make matters worse it can be completely silent .. you won't hear it until it's too late .. no detonation sounds, knocks or pings.
The condition just described is for a plug with the wrong heat range (too hot) run in an engine in good condition, correct carb etc.. the same as described above. If it is known that the correct heat rangeplug was used then other causes such as too lean a fuel mixture, timing advanced too far, etc., etc., are overheating the plug.
This is where "reading the plugs" can be used to determine all sorts of engine problems. But as mentioned it must be known that the correct plug was used to start with.
The heat range test is used to make this determination. At the outset always choose a colder plugfirst if you are not familiar with the nature of the plugs said to be hotter. That way the worst thing that can happen is the plug can foul out whereas too hot a plug can cause the problems above.
Written by Larry Brumfield, creator of the Brumfield head .
