Carb - Lean/Rich Tutorial

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For those who want the short answer: Rich means either too much fuel or not enough air. Lean means either too much air or not enough fuel.

Take your bike to a shop with an experienced mechanic and have them tune it for you, fine tuning a carb isn't something a novice should do. Okay, now that we're done with the short answer, it's time to go into more detail. This is the time where you break out the caffeine, no-doze and all the other anti-sleep aids.

Air needs to be balanced with fuel: Before we go into this entire rich/lean topic, we have to understand the basic operation principle of the engine, which requires a certain amount of air to a certain amount of fuel. We call this ratio the "stoichiometric ratio". Without getting too technical, it is about 14.5 parts of air to 1 part of fuel (14.5:1). Nobody that I know of tunes their jetting/carb to be at this ratio, in fact it's common to find engines to run anywhere between 11:1 to 14:1.

a. 11:1 is more fuel than 14:1 because there's less air per volume, hence it is richer.

b. 14:1 is thus leaner than 11:1 because there's more air and thus less fuel per volume.

So far sounds simple enough right? Basically there's only so much volume of fuel that can be mixed in with a volume of air at any given time.

Enter 2 stroke oil Okay, now that you think that all that matters is adjusting the fuel to be proportional to the air going into the engine, 2 stroke oil adds another issue that directly affects the air to fuel ratio.

I want you to imagine an empty bucket, with a permanently sealed 1 liter plastic bottle inside filled to the top with fuel. The bucket represents the total air volume possible in a given situation. The sealed bottle inside represents the amount of fuel needed to achieve a good air to fuel ratio (stoichiometric ratio). When we introduce 2 stroke oil, it doesn't affect the air volume, it affects the fuel volume.

You might be asking "why is the fuel bottle sealed"? A sealed bottle has a limit to its capacity. The orifice of the jets are a set size, therefore there's a limit to that jet size's capacity to provide fuel. When you replace a smaller jet with a bigger jet, what's happening is that you're replacing that old sealed bottle with one that is larger. The empty bucket doesn't change.

When you add 2 stroke oil, it goes into that sealed plastic bottle, so naturally since there's only so much liquid that can fit in that bottle, by adding oil some of the fuel has to get dumped to make room for that oil. Therefore, for those who use richer oil to fuel ratios (32:1 is a richer oil to fuel ratio than 50:1), you're displacing more of that fuel with oil. Now in relation to the bucket, you have less fuel to the same amount of air because the oil took some of that fuel away.

>>> !!! PROBLEM !!! <<< Now we have a bottle that has less fuel that we need because some of the fuel got replaced with oil, this is a lean condition, what am I supposed to do? How can I replace the lost fuel and keep the amount of oil I intend on using?

So what can we do? Let's just say for the sake of this example that 20% of that plastic bottle is oil, the other 80% is fuel. The total ratio requires 100% fuel to achieve stoichiometric ratio. There's no more room for fuel in that bottle, so what we need is a larger bottle that'll allow us to restore that 20% of fuel we lost. Using a larger jet or moving the needle clip down will accomplish this. By using larger jets, you increase the capacity, just like how it would by using a larger bottle. This is why a bike that uses more oil will tend to use larger jets than one that uses a lot less oil.

All things being the same (assuming using the same engine and oil), a bike jetted for 50:1 will definitely be using smaller jets than one using 20:1 oil to fuel ratio.

Spark Plug Reading Many folks use the traditional 4 stroke method of reading spark plugs to determine rich or lean conditions. I seriously advise against it. I'm sure you hear from a lot of folks that you need to find an open stretch of road so you can hold it full throttle for 1/8 to 1/4 mile then cut the motor ensuring not to let the engine idle down, then you pull out the spark plug and look at the insulator's color. This is wrong.

1. The color of the insulator can be affected by the type of fuel and oil you are using. You can have an overly rich condition at wide open with a white porcelain insulator.

2. A brand new plug will not have time to develop enough deposits to turn into the correct color.

3. Your jetting could be right on the nose, but you might be using a spark plug with too hot a heat range. The color of the insulator nose will result in a whitish color. It'll make you think that your jetting is lean so you'll end up making things rich.

4. Your jetting could be very lean but your spark plug's heat range may be too cold. The color of the insulator nose will result in a chocolate brown color making you think that everything is ok.

MIXTURE RING It's not easy to see where this is but here's one in addition to losthope's. The mixture ring is also known as the black soot ring. This is where you are supposed to look at when jetting your bike correctly. The black soot ring (mixture ring) will be thicker as it is richer, sometimes darker in color. If there's no soot ring or it's very thin (less than .5mm in thickness), it's lean. An engine that's tuned too rich will have a very thick soot ring while a well tuned engine will have a nice .5mm to 1mm ring).


Mixture Ring

SparkPlugCutawayMixtureRing 1.jpg

Mixture Ring Diagram (Black Soot Ring)

SparkPlugCutawayMixtureRing 2.gif


Question "Can you explain the need for a high amount of oil in the fuel? Obviously I understand the basic concept of lubrication, but why do some people run a higher ratio (oil to fuel) than others? Is there a way to determine the right ratio? What are some of the benefits/downfalls or using more/less oil in the mixture?

For example with a big bore kit. Kenny might run 25:1 while others will run 32:1. I understand that the 25:1 mixture will require a larger jet but what is going on inside the engine? Is there a benefit for the 25:1 mixture?"


Answer from Wheelman-111

I've goofed with - and read about - various 2-strokes for a long time. I forget where I read this, but someone will recognize the gist of it: Smart People (not me!) Testing HiPo two strokes have revealed that the more oil you add, the stronger the engine runs, up until the plugs foul which is usually down at below 20:1. . Why? I don't know for sure. I do know things slide easier the greasier they are... :oops:

But seriously folks, decreased internal friction means less power loss to overcome it. The downsides of increased oil are: 1. Increased carbon residues in your combustion chamber, 2. Wastefulness of Precious Earthly Fluids, and 3. Mrs. Kravitz's disapproving glare at the clouds of blue smoke behind your bike as you ride it over her petunia bed. Read on.

I read the Arnadanoob's Stoichiometric Primer with interest. I agree in principle, but I'm not convinced that (except for racers) the fuel/oil ratio has all that much practical effect on the air/fuel ratio, stoichiometrically speaking.

Y'see, another way to look at this is to express the fuel/oil ratio as a percentage:

50:1 = 2% (Only 98% of the liquid mixture is in fact, Hi-Test Gasoline.) 40:1 = 2.5% 32:1 = 3% ( yeah, not exact, but you get the idea... :) ) 25:1 = 4% 20:1 = 5% - Where 95% of the mixture is Gasoline.

(10:1 = Congratulations, you now own a Diesel Engine.)

Example: A theoretical oil-free Engine running pure gas, WOT, where jetting yields 14:1 air/fuel at a constant temperature and pressure - fully warmed-up and steady at max RPM. Now add the mixtures of fuel and oil from above:

14 parts of air mixed in the carb with Gas "diluted" with oil @ 0.98 =14.3:1 You can fill These in Yourself , you Lazy *^$#%@ :) 14 parts of air mixed in the carb with 0.95 = 14.7 : 1

Not such a huge difference between 50:1 and 20:1 after all, except to a racer. 14.3 to 14.7. Too lean for max power.

So now we know increasing the oil content of the fuel does indeed decrease the combustible fuel content in that mixture in the float bowl. (Does some of the oil burn? Maybe... but let's save that for another time.) Therefore it affects to the degree shown the air/fuel ratio. Note: I lack the knowledge to address the partially compensatory effect adding oil evidently has on how well the engine runs and how long it lasts. The leaner mixture should burn hotter, but then there's more oil sloshing around in there too. But if the jetting's right, it seems to work. See paragraph 1.

Presuming a fuel/air mixture of 14:1 is ideal in this racer's application, for absolute perfection he/she needs to jet up when adding oil. In order to continue wringing out every last drop of performance from this hypothetical screamer jetting up restores the ratio of air to fuel, getting it back down to 14. More total fuel mixture compensates for the oil that is substituted for gasoline in the mixture. But for the rest of us?

Scooting down Elm street to the Quickee Mart, I submit that the average rider would not know the difference between an engine jetted anywhere between 13 and 15:1. At 18:1 or more, you can tell. The kickstarter jams.

With all that said, remember the Devil :twisted: in the details. On startup, a lot of the fuel is condensing in droplets stuck the walls of the intake side. Less fuel, more cold air gets to the Chamber. That's lean. Lean means hot. It's hard to be sure the Bystarter is compensating precisely. The fact that it starts offers a clue, but take it easy heading off.

As you ride the mean streets of Anytown, you're on and off the throttle, revving up, coasting down. The engine cools when you stop for your Grape Slurpee, gets hot when you race that unicyclist. (Did you lose again? :oops: ) Two or more carb circuits "handing off" mixing responsibility to each other as the ride proceeds. Lots and lots of variables to consider. Rich might foul your plug. Lean will make you push it home. Without your Grape Slurpee. You can be too close to perfect


Lean condition summary

A lean condition can be caused by:

1. Too much air. An example of this would be going from a higher elevation to sea level which raises air density. Riding from a hot climate to a very cold climate will also do the same. In some cases the difference between day and night riding will do this as well.

2. Not enough fuel. This is mainly due to the setting in the carb (needle clip too high) or too small a jet whether it's the pilot or main.

3. Too much oil, caused by increasing the oil ratio from something like 50:1 to 32:1 without upjetting.

So to answer your question a lean condition can be caused by a lack of fuel or an over abundance of oil without upjetting.

We all know that playing with the jetting affects the air-fuel ratio but sometimes racers have played with the oil-fuel ratio like it's a 3rd jet to fine tune things as the weather conditions changed. It's just one more thing that can be done to fine tune things.

For the rest of us, the oil ratio does affect the air/fuel ratio considerably, if in fact a bike was tuned well at 50:1 and all of the sudden the rider decides to go with a different oil at 25:1 without upjetting, I can tell with 1 turn of the throttle.


Authors: Arnadanoob, Wheelman-111


Editor: mousewheels

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