In a AIR COOLED ENGINE, the spark plug center electrode porcelain color can tell you how well your engine is running or can tell you how to get it right.
Basically the plug color is telling you the combustion chamber temperature at the time you shut the engine down.
If the engine is running well and you pull the plugs while riding on the trail and you see a light chocolate brown color all is well—This would be an average of the throttle position and shut down at that time.
Stay with me,
If you are trying to jet your carbs after doing any type of mods—pipes, porting, heads, carbs, clutching or gearing, You are going to have to do this.
First lets get it down on how to do a PLUG CHOP
This is what you have to do on a test ride each time you want to test the particular change you did to the carbs.
ALWAYS do only ONE change at a time or you won’t know which one is working
First get yourself a new box of NGK spark plugs of the correct type for your machine, this testing will take quite a few NEW plugs.
Now warm up the engine in a area that you can hold the throttle in one position for at least 1 mile ( a lake is great for this) with a set of usable not new spark plugs in it.
Doing a plug chop plug chop on a snowmobile can be dangerous, So you must have your head on straight (no beer or drugs)
Right now I’m not getting into carb jetting or systems, I’m assuming you know what change effects each system of the carb—I will be doing another article on carb jetting later—Right now I’m trying to get you to know if the adjustments you are doing are working.
Say you moved the needle one clip position and your trying to get rid of a flat spot.
We want to get the engine RPM up to the throttle position that your testing (a neddle clip change works around ½ throttle)
So here we go, you have the engine all warmed up and cleaned out (from several test runs to make sure it’s safe). Replace the plugs with a set of new ones (they will be hot-use a rag to handle).
A plug chop is getting the engine to the test RPM as fast as possible and MAINTAINING that RPM for at least 1 mile with out changing the throttle position AND shutting down the engine as fast as SAFELY possible.
With the warm engine with new plugs in it, Have all your gear on, Start the engine and go to half throttle as fast as you can and hold it for at least 1 mile-try not to move the throttle—after the 1 mile you need to be on a flat smooth spot and hit the kill switch on the handle bar and lock up the track with the brakes and CAREFULLY slide to a stop. If you start getting side ways, let up on the brakes a little—DON”T roll it and hurt your self.
So we got the engine up to speed, held it for 1 mile and stopped the engine as fast as possible.
Now reach into your pocket and get the spark plug wrench you put there and remove the very hot plugs.
Congratulations, you just did a plug chop!!
Looking down the inside of the plug porcelain you are looking for a ring of color or a band about 1/8”to 3/16” wide. This is why you use a new set of plugs each time you do a plug chop, this ring will only show up one time.
If you went far enough on that plug chop ride you can get enough color to cover the whole electrode and it will look like a regular new plug with color—just go by the color—a great running engine will have a light chocolate brown color—darker is richer and a lighter color is leaner, You can adjust the jetting and try again.
I do this at low speeds to wide open, each a new set of plugs and test ride and it can take most of a day, After this you KNOW it’s running right!!!
I hope this makes sense and works for you-Remember your controlling the heat inside the engine, and that heat is coloring the plugs.