Usually, a person performs a plug chop to see the effects that a recent tuning has had on the engine. The plug can be interpreted to give information about the fuel/air ratio. It is important to realize that other factors (such as compression, weather, and carbon buildup) can affect plug readings and that chopping is only a glimpse of the goings-on inside the engine. All the same, it is a useful tool for analyzing the condition of an engine.
Performing the Plug chop
First, buy a new spark plug (used plugs won't give a good reading), but don't install it yet. Drive around for a few miles to bring the engine to full operating temperature and install the new plug. Now find a good straightaway with low traffic and run WOT for half a mile or so. Note: Some sources recommend running 5 miles or more to get a good reading. While still at WOT, hit the kill switch and coast to a stop. Do not allow the engine to idle before turning it off. The reason for this is that you want the plug to show the conditions of your engine at its hottest temperature and greatest amount of fuel/air mixture. Remove the spark plug and put the old one back in and ride home. Now you can carefully examine your plug.
Look at the spark plug under a good light, paying attention to the area deep in the plug around the base of the center ceramic (you can cut the plug with a hacksaw if it helps). A nice chocolate brown color should be about right. Death white or gray is too lean (too much air, not enough fuel), black is too rich (too much fuel, not enough air). When adjusting the fuel/air mixture of your engine, you always want to start out rich and work your way down. Running a little rich won't hurt as long as it rev's out well and isn't fouling plugs but you should avoid running lean as it leads to poor lubrication and high temperatures, which can lead to engine seizure, piston holes, and loads of other bad stuff.
Plug Chop Poetry
“Plug Chop Plug Chop Plug Chop” I cry!
“Until you learn to feel me,
You’ll have to do it by eye.”
When it's dirty and black, it's got the shits,
You have too much gas you are running rich.
When its hot and white, but looks pretty clean,
You're 'bout to blow your shit up; you're running lean.
Poetry by Harold
Plug Chop Alternatives
HFFS how far from four stroking method