I've been getting a few PMs about jetting and how to read a spark plug on a two-stroke engine. Two strokes are a slightly different beast, and are often misunderstood.
First, when it comes to reading two stroke spark plugs, forget most of the charts you see on line. Those pictures are of four stroke automotive spark plugs!
Second, always err on the side of rich, rather than lean! Also, read the next paragraph, and let it roll around in your mind while looking at the rest of this..................
Two strokes receive all of the engine lubrication through the oil mixed with the gas.
Gas mix ratios are expressed as parts of gas / parts of oil.
For example 32 / 1 or 32 to 1 means you are mixing 32 parts of gas to 1 part of oil. This provides more oil than a mix of 40 / 1 or 40 to 1.
Carburetor jets are a precise fixed orifice, they pass a certain amount of fluid. Jetting is the act of changing that set amount of fluid flow.
This is a hard concept for a lot of folks to grasp, but changing the oil to fuel ratio changes your "jetting" also. The fixed orifice flows the same amount of liquid,
but changing the make up of that liquid effectively also changes the "jetting"!
Keep this in mind, it' will come up again later. But, onto and back to spark plugs!
OK, here goes........................
These pictures will probably, (almost certainly), be different than what you usually see. Why? Because what is usually shown are four-stroke automotive plugs!
First, let me show you a few different types of plugs you will probably run into, and a few you may never see.
The first, is your "garden variety" standard spark plug, that most of you probably have in your bike:
You may also see the next kind of plug, the fine wire electrode spark plug. Most of these will be made with an "exotic" metal like platinum, gold, copper, etc. and may be referred to as a "racing plug". They are usually priced a little higher, generally fire better under adverse conditions, but really don't make a noticeable difference over the "standard" plug
The next plug is a multiple ground electrode spark plug. These too are usually a fine wire / exotic metal plug. No real difference over a "standard" plug, but a new twist to improve on firing under adverse conditions.
The last plug used to be a very popular "racing plug" when CDI ignitions first came out about 25 years ago. If you do much "vintage" bike restoration, this type came stock on the early Yamaha and Suzuki MX bikes. It's a surface gap or retracted gap spark plug.
Another thing to watch is reach or the length of the threaded shank of the spark plug. Generally there is three different reaches of spark plugs. If you have any doubt over what you need consult your owners manual, shop manual, or worst case, pull the head and look! Time and money wise, you are way ahead doing that than breaking a piston crown, and risking a bent rod! Here's an example of the different lengths.
OK, lets look at the kinds of things you'll see while getting your jetting sorted out.
The first picture is of a correctly jetted plug. The center electrode and the bent side ground electrode should be a nice medium to light tan. The outer threaded shell on this plug is just slightly wet, and could use just about a 1/4 turn less air screw, but is looking pretty good over all. If your's looks looks like this, you are 95% there!
Next, let's look at some unhappy plugs, some that you are quite likely to see. The first plug is from a very rich engine, that also need the air filter cleaned. Not only is it a wet / sooty black, but it shows a lot of fine dirt that has gotten sucked through the filter. The second plug is also too rich, but is a dryer "sootier" black. I's closer to the correct jetting, but still probably at least 2 sizes too big on the main jet, and could probably use the needle dropped one clip.
The Next plug you may see too, but not for long. Why? Because it's lean, and too lean on a two stroke means too much heat is being generated. Note: this carb is off all over, the outside threaded shell shows it to be too rich on the idle or pilot jet circuit, but lean on the main jet circuit. You can't really fix this by adding oil (remember that paragraph I asked you to read and keep thinking about) because the extra heat will cook off the oil, resulting in the second spark plug picture. The one coated with melted aluminum from the top of your piston. If this piston doesn't seize to the cylinder wall, there will be a hole melted through the top of the piston, and the bike looses all compression.
Something else to consider; plugs have a heat range, a rating that tells you how much heat transfer or retention the plug is rated at. This will also affect your jetting, and give you one more suttle tool to work with.
NGK and Champion plugs have an oposite heat range number system.
NGK Heat Range: Lower number is hotter, higher number is cooler.
IE: BR8XX is hotter than a BR9XX, and a BR8XX is colder than a BR7XX.
Champion Heat Range: Higher number is hotter, lower number is colder.
IE: N2X is hotter than a N1X, and a N2X is colder than a N3X
Heat Range Crossover:
Common NGK Plug Types:
BR8ES or BR8EG = Standard Plug
BR8EIX = Iridium Plug
BR8ECM = Compact (shorter and easier to change)
BR8ECMIX = Iridium Compact Plug
When making jetting changes; first, settle on an oil to gas mix ratio and stick with it! It really doesn't matter whether it's 32, 40, or 50 to 1. Pick a good quality oil you trust, follow the manufacture's mix ratio, then worry about your jetting. Why?
Adding more oil to your gas makes the mixture leaner (more oil and less gas), subtracting oil from your gas makes the mixture richer.
For beginnings, let's assume you motor is jetted correctly. Now, lets assume you jetted that bike using a 38 to 1 oil mix. If you change that mix to 32 to 1 you have leaned out the bike (more oil means less gas) if you change to a 40 to 1 mix you have richen up the bike (less oil means more gas).
When you get your jetting close to that 95% spot, making any radical change can push you over the line from safe and clean to "push it home"..........................
Let me know what you think about this, and maybe I'll do a jetting "how to" write up next if any one is interested.