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Common mis-conceptions about compression/head design and fuel requirements as it relates to a 2 stroke internal combustion engine

First off: This is NOT to be considered a “Covering EVERY Scenario” or “Lesson in Science” type article. It will, most likely, have some “holes” in it that could be seen as incorrect under certain scenarios.

This article is being written to touch on certain misconceptions regarding this area. It will be “Generalized” and NOT all encompassing.

This will, most likely, be an ongoing discussion as more data and topics surface. It is difficult to recall everything on the spot.

It will be based on Theory, Reality, Direct Experience, Automated Testing, and Common Sense. All topics and references will have been “realized” using one of the above at some time. NONE will have been “fabricated”.

If you do not agree with any of it, please ask yourself if you have DIRECT EXPERIENCE, by “Direct”, it is meant-> 100% direct as the example you disagree with, NOT 99% and not lumping , components (like cylinder head design or other) into one general “bag”.

NOTE: ALL references to Compression Ratio will be full stroke, uncorrected

OK.. let’s get started….

2 Stroke Cylinder Head Misconceptions:

1) Cranking compression via compression gauge will tell you what octane your engine requires in order to run safely…   100% FALSE!

The cranking compression will not tell you much of anything about the required octane. Cranking compression is a good tool used to compare a previous, known, value to a current value in order to tell if anything has changed. This comparison is good for a “health check” of an engine. Again, not real useful in terms of determining the “required” octane for any engine.

2) ALL 10:1 UCCR compression ratio heads (or any other compression ratio) will function the equally on the same engine. 100% FALSE! 

This could be one of the largest mis-conceptions out there. Let’s break it down: 10:1 is a ratio of the volume trapped at TDC as it relates to the volume of the cylinder and head together.  For example, if you have a 100cc engine, a 10:1 compression ratio head would be 11.117cc-> 100cc+11.117cc (full stroke cylinder volume) / 11.117cc (head volume at TDC).

As you can see 10:1 will always be the SAME volume given the same engine.  What will NOT be the same as how that volume is geometrically accomplished.

There are infinite geometric designs that can arrive at 11.117cc volume.

This is where the fun begins! The same VOLUME head will yield the same compression ratio but can also yield VERY different results in terms of how it performs when installed on the engine.

The geometric design of the head is critical as to how it performs. A simple .001” change in a certain area can have a huge influence on its “effectiveness”.

Back to the misconception… Brand “A” 10:1 head can, and usually will, perform VERY different from Brand “B” head. 

Think of fast food hamburgers. If you have a double cheeseburger from one fast food chain, it will/can taste totally different than a double cheeseburger from a different fast food chain even though they have basically, the same ingredients. SAME with Head design.

3) If you raise the compression on an engine it will ALWAYS require more fuel to run safely… 100% FALSE!

This is a loaded subject.  It also relates, heavily, to #2 above. It is also going to be difficult to explain. Let’s give it a go anyway.

2 Stroke head design is a huge player (as is exhaust, ignition timing, piston, reeds, and porting etc.) in the tuning and performance of any 2 stroke engine. They all work together.

Changing ANY of these variables can have a pronounced effect on performance (HP, Torque, Over-Rev, Timing, Heat etc.) and fuel requirements (octane and volume etc).

Suffice to say, they are all “Shaking Hands” or “Punching Each Other in the Face” depending on the change made.

This article will be limited to Head Design but wanted to mention that there are other “players”, directly, involved.

Fast combustion is what is always desired. The faster we can complete the combustion process, the better. The head design has large influence on how fast this process completes and what ignition timing is optimum. Advanced ignition timing is not desired. The later we can start the ignition process and still have a good rod angle ATDC, the better. Again, the head design is has a large influence in this.  This area can become very detailed discussion. We will stop here for now.

Head Efficiency or Combustion Efficiency is the measure of the head’s ability to convert the supplied Air/Fuel into useful energy.  Obviously, we want the highest efficiency we can. Of course, there are exceptions to this as well where we do NOT want a highly efficient head ie. A tuned exhaust system that requires more energy to function better… but that is discussion for another time.

For the purpose of this article, we want the most efficient head design possible.

This, directly, relates to how much fuel/air (F/A) is “needed” to be supplied to the head. 

When the head is more efficient, it is converting more F/A mix to useable energy. Does this require MORE Fuel to accomplish? Not always. Anything that is more efficient usually requires less to get the same or better result. This is the definition of efficiency. But, with head design it is not always that simple.

Whether or not your engine requires more or less fuel when adding a head with a higher combustion efficiency directly relates to how good or poor the previous head design was and how the fueling was set up.

For example..  2014 and newer KTM 250 and 300 Enduro or SX OEM Head. 

This head design is so inefficient that it is plagued with a high frequency of misfires. These misfires manifest themselves as a rich running condition when, in fact, the F/A supplied is close to correct with stock jetting.

The misfires are so prominent that most will lean the jetting to lessen this rich condition (caused by the head’s inability to combust). It stands to reason, if you have a head that is struggling to combust the given F/A mix, you will have residual (non-combusted) F/A mix “lingering” in the engine and exhaust that will be combined with the new F/A charge and push it to a F/A mix that is too rich. Of course, this does NOT occur every stroke, but is frequent.  

Leaning the carb mix via jetting alterations will provide less Fuel and, in turn, lessen (not eliminate) the frequency of the head misfires. Of course, those times (frequent) when the head is NOT misfiring, allow for a lean condition. This lean condition will create added heat and will lessen power output. If left too long it can damage your engine.

Point being.. You are treating the “symptoms” of the problem vs. addressing the “source” (head design). You end up with a “compromise” with less power BUT ,possibly, a “cleaner” running engine. So, you feel you have fixed your issue.

If you address the source of the misfires (head design not rich jetting) you get rid of all the symptoms as well. This is the better solution. It is like having a cold. You can treat the runny nose (symptoms) and feel better OR you can get rid of the cold (source) and the runny nose goes away with it.

Now, how does this relate to installing a highly efficient head in terms of fueling?

In the case of the KTM 250 and 300, like mentioned, the head was the real culprit of the rich condition and the leaner jetting was a compromise to lessen the misfires. The engine required the larger jetting spec, but the poor head design limited the engine’s ability to utilize the required fuel.

When you run in a leaner state, you, generally, will make less power even if the engine is running cleaner. The engine requires sufficient amount of F/A in order to produce the maximum power.

They can run fine lean, they are just down on power and you would not realize this unless you compared to one that was not running lean. You don’t know what you don’t know!

When you install an efficient head, like the RK Tek Head, you do not have this frequent misfiring issue because the design of the head helps to prevent it.  With the higher efficient head design, the engine can benefit from the larger F/A volume and produce more Torque and HP.

How about fuel injection?   With fuel injection, you are only as good as your Fuel Mapping, Injector Firing, and Fuel Pump Pressure. If any of these are “out” you can have a poor running engine. It can be “lean poor” or “rich poor”.

What happens when you add a more efficient head to a fuel injected 2 stroke? Well… There are many scenarios. Suffice to state that the head designer better have some good knowledge of how to accommodate the inner-workings of 2 Stroke FI. With this knowledge, nice gains can be had with no fuel alterations or controller required.

HOW? you ask?--> It all reverts back to how the exhaust, head, ignition timing, and reeds “hand shake”.

Knowing how these interact with each allows the head designer to “compensate” in the design to allow for more power without upsetting the “balance”.  This is a very complicated topic.

It is often the case, that the design that works best for a carbureted induction engine will not be optimum for a FI engine.

Testing is required to find which design can keep all the other players “happy”.

4) Air Fuel Ratio Pre-Combustion can be measured, and is the same as AFR measured Post-Combustion.

There is a huge mis-conception that  Air/Fuel Ratio O2 sensor readings (in the pipe AFTER combustion) have a direct (one to one) relationship with the Air/Fuel Ratio reading Pre-Combustion (Carb to Head Path).

O2 or Lamba sensors measure the amount of Oxygen present in an exhaust sample and relate/compare this reading to a known reference oxygen level (Usually the ambient air).

While these readings can provide valuable information about POST Combustion efficiency and ratio, they do NOT tell you about the F/A ratio PRE-Combustion.

It relates back to #3 above. How efficient is the head? If you have a head design that is very efficient, you will have a more complete combustion process resulting is a “better” AFR shown on the exhaust sensor.

What this sensor reading does NOT tell you is what the head had to work with to create this output. It also does not tell you how “optimum” you are because you have not pushed the head to its limits (ie. Adding/Subtracting more fuel or air) to determine what it can efficiently combust. This is controversial. You can have a 13:1 AFR measured.. add some fuel or air (or both), and STILL have a 13:1 measured AFR. This will largely depend on the head’s design and how it works with the other primary engine components. It is like stressing a frayed rope., you really do not know when then rope is going to break until it does.

Point being, there are many variables in play here, including the limitations and design of the AFR measuring device, itself.  Suffice to say, that measured AFR is NOT the “End All” and can lead you astray if not careful.

With any carb system, unless the head is increasing air-flow (possible), fuel flow into the engine will remain near constant for a given RPM, heat, and load EVEN if the AFR reading changes. What, really, has changed the AFR reading is the head’s ability to combust a higher or lower percentage of what it was given.

If the head is very efficient, usually, LESS F/A is required for the same output. Again, this is all relative and not always the case.

For example, on the 2000-2012 Ski Doo 800 engine. We were able to increase the compression, significantly, and also reduce the main jet 5 sizes. This resulted in a large power increase and better fuel economy. This was due to the OEM head design being very inefficient and hot.  The OEM design needed extra fuel to keep combustion temperatures in a safe range. Without this added fuel the OEM head would run too hot and detonate. It was, essentially, cooling temps with fuel. The RK Tek Head design focused on increased combustion efficiency and minimizing hot spots within the chamber. This allowed for a much better and safer running engine.

5) Is Higher Compression required to extract more power? In short NO!

One can easily create more power using the SAME compression ratio as before but using a better design. How much more power depends on how much better the design.

Increasing compression (leaving all else the same), is always good for increased performance but it can come at a cost unless you optimize the design and “hand shaking” that is happening with the other primary engine components.

IF you optimize these relationships you can, successfully, increase the compression ratio without any compromises (gains across the board).

It is, usually, never as simple as a head “shave”.  However, while this can be effective, it, usually, is not and comes at a cost. Robbing Peter to pay Paul type scenario.

It ALL reverts to the HUGE complexity of the 2 stroke engine and how ALL primary components interact. 

‘Nuff for now.. Already way too long!! 

Does your carburetor REALLY have Altitude and/or Temperature Compensation?

Carburetion and how the Carburetor "meters" the Fuel and Air Mix. What is required (component wise) for elevation and temperature control

Due to the large amount of phone calls regarding this topic, it is prob a good idea to post up some "information" regarding this. highly debatable, subject(s).

Not going to get real indepth but just touch on a few basics that constitute what is required in order to have true elevation and temperature control with ANY carburetor.

A few "concepts" on carb function: Again, not 100% encompassing, just basics.

1) ALL carburetors function on a pressure differential (Delta P) between the Carb Float Bowl and Engine.

2) This Delta P is what will,largely,determine how much "signal" is present to "pull" fuel up the needle jet into the engine.

3) Larger Delta P will equal more fuel flow and vice versa.

4) The engine will always be the low pressure side and the carb will always be the high pressure side.

5) We can consider the engine side a constant vacuum in terms of pressure (even though it varies)

6) The Carb float bowl pressure will vary with elevation. As you raise in elevation, the atmospheric pressure will lessen (usually) and the pressure of the float bowl will lessen as well.

7) Lesser ATM Pressure will equal lesser Delta P and equate to less fuel flow.

8) IF your carb float bowl vent lines are vented ANYWHERE but into their own dedicated pressure chamber, your float bowl pressure WILL be at atmospheric pressure (or very close to it)

9) ANY and all Carbs will lessen flow as you raise in elevation. Meaning... ALL carbs have a built in elevation control simply as a result of its design and function.
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OK.. Now that the "Basics" have been touched upon.. let's dive into what it truely takes to have control over elevation and temperature:

1) VERY SIMPLE--> Control Delta P and you can control the fuel flow!

How do we control Delta P?

You can control Delta P via altering engine side pressure or carb side pressure. Simple as that.

It is possible to control either but controlling the carb side is WAY easier than controlling the engine side.

How does one control the float bowl pressure? The only sure fire method to control the float bowl pressure is to have it vented into it own dedicated pressure chamber.

This method has been used in the snowmobile world (huge elevation and temp changes) for many decades!

They would connect ALL the float vent lines together and connect them to a dedicated pressure chamber that is controlled via the ECU and an Atmospheric Baro Sensor.

Basically, they would sense the ATM pressure (baro sensor) and adjust this chamber volume based on this sensor.

They also had a temp sensor that would add to the mix and adjust even further using the temp sensor's input. NOTE: this was a "programmed" function based on sensor inputs and the mechanical circuitry to implement it.
It required dedicated electronic and mechanical circuitry. If ANY of these circuits failed (and they would occasionally) the jetting got "incorrect" in a hurry.

Worked pretty well. We could run the SAME jets at 10,000ft that the sleds at sea level were running. Pretty impressive!

So... How do you know if you have elevation and temperature control with your carb? Simple--> does it have a temp and baro sensor and a dedicated float bowl pressure chamber? IF it does NOT, then you do NOT have compensation outside of the built in mechanical compensation present with ANY carb.

OTHER POINTS OF INTEREST--> It is ,widely, "believed" that the atomized fue/air mix (F/A) (as it leaves the carb to enter the engine) is in its FINAL state to be used for combustion. 

This is NOT the case. 

The F/A mix is in an atomized state as it leaves the carb to the engine.. This F/A mix will "phase change" into a Vaporous F/A Mix once it enters a lower pressure environment (ie crankcase) and/or encounters some heat (ie engine). 

The engine wants a vapor for combustion, NOT atomized F/A Mix.

We ask ourselves, does this carb or that carb do a better job of atomizing the F/A?  We think that if one carb does do a better job that it will produce better results. Maybe it will, maybe it will not.  Point being... Since the atomized F/A Mix is NOT its final form for the best combustion and it must phase change to a vapor (gas), does it matter what form leaves the carb since it must phase change anyway? 

Think about 2 people climbing a mountain but each taking a different path to get there. Once they are BOTH standing on top of the same mountain, does it really matter what path each one took to get there? 

Point being-->  The F/A Mix needs to phase change. Whether this phase change happened more "easy" or "difficult" should not matter as long as it has occurred! 

This is one reason why Fuel Injection (as its own entity/device) does not provide any more power over a carb induction system..

The required end result is still a vapor and FI does not supply a vapor as it leaves the injector.  The phase change occurs inside the engine no matter how "atomized" the inducted F/A charge was or was not. IT STILL WAS CHANGED TO A VAPOR! 

SUMMARY: The type, model, style, color etc etc. of the device supplying the atomized F/A Mix is not nearly as important as the engine's ability to convert this mix to a Vapor/Gas.  

2 STROKE ENGINE AIR/FUEL RATIO: What it means and how is it determined?

Air/Fuel Ratio (AFR) seems to be widely mis-understood as to what it actually represents and how a non “ideal” AFR effects the running of the engine. 

Let’s try and break it down into VERY simple terms and ,hopefully, gain a better understanding of it and other related areas where it has an effect.

OK, found this definition on the internet and it is fairly accurate so, let’s use it for now. “Air–fuel ratio (AFR) is the mass ratio of air to fuel present in a combustion process such as in an internal combustion engine or industrial furnace. ... If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich.”

In simple terms, it is the ratio of Air to Fuel in a combusted charge.

Think of mixing your oil in your 5 gallon fuel container. If you mix at 50:1 that is 50 parts fuel to 1 part oil.

KEY NOTE:  AFR is a RATIO and is not referencing a VOLUME. There are an infinite number of possibilities of Air-Fuel Volumes that can have the SAME Air-Fuel Ratio.   Just like mixing your fuel and oil. If you mix 10 gallons at 40:1 it is much more fuel than mixing 1 gallon at 40:1 BUT the 40:1 RATIO remains the same.

AFR for a 2 stroke engine can vary greatly and the engine can still run well. There are MANY variables that determine what the “best” AFR is for a particular engine and its set up. What is ideal (AFR) for one engine may be problematic for another, Application and Power output play a part in this, as well as, many other variables. The purpose of this article is not to “dive in” to all those variables (maybe later) but to simply gain a more comprehensive understanding of AFR and how it relates in an engine.

What is the AFR range for a good running engine? As stated, above, it varies. Generally, this range will be from 12:1 to 15:1 for Gasoline Engines. Again, at 12:1 AFR that is 12 parts Air to 1 part Fuel.

For this article, we will focus on a Naturally Aspirated Engine equipped with a slide valve carburetor and an air-box that is exposed to the atmosphere. Typical motorcycle set-up. We will also assume that the fuel octane requirement is adequate for the engine’s design.

How is the Air and Fuel supplied? Air is supplied via the air-box and inlet track. Fuel is supplied via the carburetor circuitry.

Air passes through the carb body causing a venturi effect which, in turn, pulls raw fuel into the incoming air stream creating a stream of Air and Fuel Mixture (AFM).

This AFM enters the engine when the pressure on the engine’s intake track is less than the atmospheric pressure found at the carb outlet. This pressure differential (Delta P) is mandatory if any AFM is to enter the engine.

This AFM is atomized before it enters the engine and becomes even more highly atomized (vaporized-phase change)once inside the engine.   

This, vaporized, AFM is directed towards the combustion chamber (head) where it will, hopefully, ignite and combust. Remember, this AFM has an Air-Fuel Ratio (AFR) associated with it.

This AFR will determine how completely it will combust and how much energy will be delivered during the combustion process. Yes, there are other “players” involved besides just the AFR, but we are keeping it simple for this article.

The combusted charge is expelled and the whole process repeats.

Now, that the process has been defined (in the most simplistic manner). We can talk about what occurs when the AFR of the AFM is not ideal or close to ideal.

RICH Air-Fuel Ratio: This would represent more parts fuel vs. air than is needed.  

With a Rich AFR, the AFM may not completely combust. Incomplete combustion means that you have not extracted the max amount of energy that was available for a given AFM.  While this is not desired, it is not always a bad thing. Incomplete combustion, a result of a rich AFM, does have some “perks”. 

One perk would be that any un-combusted fuel can act as a cooling agent to lower internal engine temps which can help keep the charge density higher.

Another perk is that it can lower the exhaust temperature which will cool the pipe. With a 2 stroke pipe, a cooler pipe will alter its effective “tuned” RPM and this can greatly aid performance when operating at a lower RPM than the pipe was designed.

There are some other “perks” but we will stop for now and move on.

Rich AFM can manifest itself in a few different manners:

1)A very common example would be to have a stuttering or hesitation in the engine’s running. NOTE: This is NOT just a “NOISE” but a REAL runability issue. Do NOT chase NOISES!

2) Massive mis-fires and spark plug fouling.

3) Extreme jetting sensitivity. Engine can be overly sensitive to temperature and elevation changes.

LEAN Air-Fuel Ratio: This would represent more parts air vs. fuel than is needed.

With a Lean AFR, the AFM may also not completely combust.

Lean AFM will produce more internal heat. This added heat can be un-harmful for short periods. If you allow this lean AFM to continue, the heat will build/grow onto the engine’s surrounding components and it WILL cause problems including severe engine failure!

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Lean AFM can manifest itself in a many different manners. Here are a few:

1) Narrow Power Band: The effective power-band can be shortened causing abrupt power delivery. This also lessens the lower RPM power output. If your engine comes into it power-band very abruptly, with little power before (ie narrow power-band, big hit), it could be due to a lean AFM.

2) Engine over-heating: Like stated above, lean AFM raises internal engine temps. When your engine is hotter, everything else is hotter. It is not uncommon for water cooled engines to over-heat as a result of a lean AFM.

3) RPM run-on: The lean AFM’s added heat input can cause a higher RPM that is slow to return to its correct RPM. This is sometimes referred to as a “Hanging RPM”

4) Detonation: Please see the article here:  https://www.2strokeheads.com/index.php/site-map/articles/80-technical/91-exposing-the-myths-of-high-octane-fuel-and-the-definition-of-detonation

5) Pre-Ignition:  Not really a direct result of the lean AFM, but the lean AFM can certainly be a contributor to pre-ignition. Pre-ignition occurs when a point in the engine becomes so hot that it becomes a source of ignition and causes the fuel to ignite before the spark plug fires. This, in turn, may contribute or cause a detonation problem. Pre-ignition is initiated by an ignition source other than the spark, such as hot spots in the combustion chamber, a spark plug that runs too hot for the application, or carbon deposits within the combustion chamber that have been heated to incandescence by previous engine combustion events.

The phenomenon is also referred to as 'after-run', or 'run-on' or sometimes dieseling, when it causes the engine to carry on running after the ignition is shut off. 

Excessive carbon build up or any sharp object present during the compression stroke can onset pre-ignition. This is much more common a 4-Stroke Engines vs. the 2-Stroke Engine.

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POINTS of INTEREST:

1) All AFM is supplied via the intake track into the lower end of the 2 stroke engine. If there is any residual AFM within the engine from the previous combustion event, it can richen the AFR of the AFM. It can also dilute the new AFM charge lowering its potential energy.

2) Detonation, most always, requires a load in order to occur. Meaning… if you think you are hearing detonation under a “no-load” running condition (i.e. idle), you are probably mistaken.

NOTE: There can be a very brief detonation event after you "roll-off" the throttle from a heavy loading. This event is very short and you May hear a few "clicks" of detonation.

3) Air Leaks:  Added air via a “leak” is common in creating a lean AFR condition. These leaks can come from a few areas. Some of the more common areas are: Cylinder Base Gasket, Reed Gasket, Reed Boot, Air Filter Seal, Air Filter Boot and even a cracked exhaust or cracked cylinder. These are just a few examples.

4) Air Leak “Weight”: This is an important concept.  Given you have an air-leak, the effect that leak will have will be directly related to the volume of AFM. Let’s break it down further: Given a 0.5mm size air leak hole, how much “added” air that can be introduced via that 0.5mm hole is relative to its size, the pressure surrounding it, and RPM. This is also relative to the AFR and volume of the incoming AFM.  For example:  During idle there is a VERY small volume of AFM entering the engine. A 0.5mm air leak hole, can have a large effect on the AFR of the AFM because the volume is so small.  That same 0.5mm air leak hole would have much less effect on the AFR of a larger volume of AFM like you would have at high load and high RPM engine running. So, air leaks will have a greater effect when the engine is under low load and low RPM (i.e. Idle, Low Throttle Operation).

5) Once the AFR of the AFM has been set (pre combustion phase) it is set. Meaning, the combustion chamber is going to try and ignite and combust  WHATEVER AFM is trapped in it. It does not care!! It will do its best to complete the process regardless of the results.  The combustion chamber design will not cause a supplied AFM to go lean or rich. The combustion chamber does not alter the AFR. If it is supplied a rich AFM, the engine will have a rich AFM combustion event, if it is supplied a LEAN AFM the engine will have a lean AFM combustion event.

Side Note: having a more complete and energetic combustion process can increase air-flow within the engine. This is where it gets a bit complicated. When everything is working better you can have a higher delivery ratio and the pipe can deliver more charge as well. In short... when all internals and externals are "jiving", it can require more fuel via the carburetor.

6) AFR’s are generally, leaner at low load RPMs and richer at the higher loads and higher RPMs.

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MORE  "NOTES"

All these "Plug reading and color charts" are from the 1970's and early 1980's and MANY of them are 4 stroke based. This is very important. Also, most bikes were Air Cooled.

1st off--> Using 4 stroke "tuning theory" or "indicators" can get you heading the wrong direction when applied to a 2 stroke.

This is super important because that is what is happening with much of the advice given out on the media.

2 Stroke tuning is VERY different from 4 Stroke tuning. In many cases 180 degrees opposite.

While AFR tests/readings (via dyno or real-world) CAN BE somewhat relevant, they are only relevant if tested (ie probe location and type) is relevant and most tests do not have this correct..

When you see a dyno chart and everybody is all concerned with the AFR reading.. BE CAREFUL!! AFR readings are VERY subjective and the "ideal range / value" for best power will vary with other factors and may not be what you think. That is another subject.... Point being.. don't think that when you see an AFR in the "correct" range that the engine will be producing its best power as a result..

OK back to the charts...>>
In the 70's and 80's ,pretty much all of the USA had the SAME FUEL. So if you got 91 octane in NYC and in LA, it would be very close, if not, to the same "blend" and most of it would contain lead. So we all were running the same fuel and it was probably leaded.

There was probably less than a dozen 2 stroke oils being used. Most of these oils were, mineral based and very similar in chemical make up. There were some synthetics and bean oils.

SO, in the 70's and 80's (when these plug reading charts surfaced), most all 2 stroke engines were using the same fuel and the same oil or a derivative thereof.

Like "anything" that is "common", you can come to a consensus on certain things based on this commonality.

This is what these charts are based around--> Common fuel and common oil.

Basically, if we are all running the same "mix" we can make some conclusions after the fact.

We were all running the same mix and once you got a 2 stroke engine tuned, you could pull the plug and say "Hey, this engine is running great and the plug is "Brown".. Another rider ,2000 miles away ,can do the same and say "My engine is tuned well and my plug is also Brown"
Same scenario for Rich and Lean conditions as well.

So, you can get the point??? Charts were formed as a result of this commonality among mixes and engine performance.

Fast forward to the 21st century (and even before)... The fuels we use are no longer leaded, and vary from block to block , let alone, from state to state. We have 100's of different oils and additives (both in the oil and the fuel). Plug technology has progressed significantly, as have ignition systems just to name a few!

So, Why would one use 1970's criteria to judge 2021 engines?

Be careful, when you hear your "tuner" tell you that you are looking for a certain "color" on the plug in order to determine how close the jetting is.. That line of thinking simply does not hold true anymore and has not for decades.