MOTORCRAFT
AWSF-32C / stock heat range - copper plug
AWSF-22C / 1 Heat range colder - copper plug (Truck application, knock sensor)
AWSFA-12C / 2 Heat range cooler - copper plug
AGSF-12FM1/ (fine wire electrode) 2 steps colder (.044 gap) (Stock L plug)
AGSF-32C/ Recommended for FRPP heads (Full thread) stock heat range

NGK
PART# / HEAT RANGE
TR-5 / / 1 step hotter than stock
TR-55/ 3951/ Stock Heat Range
TR-6/ 4177/ 1 Step Colder
BP7EFS (PN - 3526) BR7EF (PN - 3346) two steps
R5724-8 (PN - 7317) two and a half steps
R5724-9 (PN - 7891) three steps
R5724-10 (PN - 7993) four steps

AUTOLITE
PART# / HEAT RANGE
764s / Stock
103s / 1 heat range cooler
AR94 / 2 heat range cooler
AR93 / 3 heat range cooler
AR92 / 4 heat range cooler

CHAMPION
PART# / HEAT RANGE
YS-59C / -3 or -4 below stock heat range

DENSO
DENSO (.38 gap)
PART# / HEAT RANGE
IT16 / stock heat
IT20 / -1 heat range
IT22 / -2 heat range
IT24 / -3 heat range
IT28 / -4 heat range

The old guidelines for reading plugs that applied back when we were running leaded gas are fairly meaningless with today's reformulated, oxygenated, MBTE rated sorry excuse for motor fuel which we have to live with. There is a way to read plugs today, but don't think you are going to see that nice coffee-with-cream color we used to strive for; it ain't gonna happen! Gasoline as we now know it burns sooty. Remember when the exhaust pipe on your car used to have that cigarette-ash gray color if it was running properly? Go look at the exhaust pipe on your car or bike right now. Black, huh? This doesn't mean it's running too rich, it's just the way the stuff burns.
What you want to look for on the spark plug is a faint black or dark gray ring of carbon on the white center insulator (which surrounds the center electrode). You will have to look closely, it's dark up in there. The best thing to use is a lighted magnifier, it is hard to see. The richer the mixture is, the closer the carbon ring will be to the tip of the insulator.
With an ideal mixture, the carbon ring will be way up at the top (not tip) of the insulator, up inside the recess of the spark plug. Now, what you want to do is called a "plug chop" to see how the mixture is at a given throttle setting. Run the car to get it up to operating temperature then run at the throttle setting in question for a minute or so. Without changing the throttle position, kill the ignition. The plugs will show how the engine was running at the moment you killed the ignition.
If you ride around, go home, and park in the driveway and then look at the plugs, they will tell you only how it was running when you shut it off, which was at idle. Ideally, you should first get the carbs dialed in at full throttle; if you can pull a long hill with the throttle wide open, this is a good way to check it. This wide-open throttle check will tell you if the main jet is correct, since at full throttle you are running primarily on the main jet. You can go larger or smaller on the main jet as needed, but this needs to be dialed in first, otherwise you will be chasing your tail with needle clip changes, etc.

 

Mechanical Damage May be caused by a foreign object that has accidentally entered the combustion chamber. When this condition is discovered, check the other cylinders to prevent a recurrence, since it is possible for a small object to "travel" from one cylinder to another where a large degree of valve overlap exists. This condition may also be due to improper reach spark plugs that permit the piston to touch or collide with the firing end.	Oil Fouled Too much oil is entering the combustion chamber. This is often caused by piston rings or cylinder walls that are badly worn. Oil may also be pulled into the chamber because of excessive clearance in the valve stem guides. If the PCV valve is plugged or inoperative it can cause a build-up of crankcase pressure which can force oil and oil vapors past the rings and valve guides into the combustion chamber.	Overheated A clean, white insulator firing tip and/or excessive electrode erosion indicates this spark plug condition. This is often caused by over advanced ignition, timing, poor engine cooling system efficiency (scale, stoppages, low level), a very lean air/fuel mixture, or a leaking intake manifold. When these conditions prevail, even a plug of the correct heat range will overheat.
Insulator Glazing Glazing appears as a yellowish, varnish-like color. This condition indicates that spark plug temperatures have risen suddenly during a hard, fast acceleration period. As a result, normal combustion deposits do not have an opportunity to "fluff-off" as they normally do. Instead, they melt to form a conductive coating and misfire will occur.	Pre-Ignition Usually one or a combination of several engine operating conditions are the prime causes of pre-ignition. It may originate from glowing combustion chamber deposits, hot spots in the combustion chamber due to poor control of engine heat, cross-firing (electrical induction between spark plug wires), or the plug heat range is too high for the engine or its operating conditions.	Normal Combustion deposits are slight and not heavy enough to cause any detrimental effect on engine performance. Note the brown to greyish tan color, and minimal amount of electrode erosion which clearly indicates the plug is in the correct heat range and has been operating in a "healthy" engine.
Gap Bridging Rarely occurs in automotive engines, however, this condition is caused by similar conditions that produce splash fouling. Combustion deposits thrown loose may lodge between the electrodes, causing a dead short and misfire. Fluffy materials that accumulate on the side electrode may melt to bridge the gap when the engine is suddenly put under a heavy load.	Splash Fouled Appears as "spotted" deposits on the firing tip of the insulator and often occurs after a long delayed tune-up. By-products of combustion may loosen suddenly when normal combustion temperatures are restored. During hard acceleration these materials shed from the piston crown or valve heads, and are thrown against the hot insulator surface.	Detonation This form of abnormal combustion has fractured the insulator core nose of the plug. The explosion that occurs in this situation apples extreme pressures on internal engine components. Prime causes include ignition time advanced too far, lean air/fuel mixtures, and insufficient octane rating of the gasoline.
Ash Fouled A build-up of combustion deposits stemming primarily from the burning of oil and/or fuel additives during normal combustion ... normally non-conductive. When heavier deposits are allowed to accumulate over a longer mileage period, they can "mask" the spark, resulting in a plug misfire condition.	Carbon Fouled Soft, black, sooty deposits easily identify this plug condition. This is most often caused by an over-rich, air/fuel mixture. Check for a sticking choke, clogged air cleaner, or a carburetor problem - float level high, defective needle or seat, etc. This may also be attributed to weak ignition voltage, an inoperative preheating system (carburetor intake air), or extremely low cylinder compression.	Worn This plug has served its useful life and should be replaced. The voltage required to fire the plug has approximately doubled and will continue to increase with additional miles of travel. Even higher voltage requirements, as much as 100% above normal, may occur when the engine is quickly accelerated. Poor engine performance and a loss in fuel economy are traits of a worn spark