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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!


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.



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.



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.


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