I love this topic and its one I am still learning the ins and outs of, but its also a rabbit hole is some ways. The theoretical ration of gasoline and air (note I did not say Oxygen) is around 14.7:1. Meaning you need 14.7g of air to every gram of gasoline to burn all of the molecules completely. So basically Lean is less fuel to air and Rich is more fuel to air.
To really understand Rich and Lean, you also have to understand how your carb works or how it delivers fuel as well as what type of engine you are running it on (4T or 2T).
4T’s (4 strokes) are easier to deal with, because you are separating the lubrication from the fuel delivery unlike in 2T’s (2 strokes) where you have to add oil the the fuel in order to lubricate the engine.
If your engine ran at a constant speed it would be easy to set the carb for whatever the current condition was and forget about it until conditions changed. However we run them at varying speeds and that complicates things. Lambda is a great way to measure fuel/air mix on 4T because you don’t have oil residue in the exhaust that would foul the sensor like you do on a 2T. With oil in the exhaust on 2T’s its better to use EGT (exhaust gas temperature) sensor that is not always straight forward, but is a very useful tuning tool. It is, however most useful at high revs and peak values.
All carbs are simply a means of dispersing atomized fuel into the air charge entering the engine. All do this by the passing of air over an opening connected to the fuel source. Think of it like blowing across the top of a straw in glass of water. You are going to draw water up the straw and tiny droplets will blow off the top. Blow softly and little or no water comes out, blow hard and more water comes out. The challenge is getting the Ideal Fuel to Air mix entering the engine at any given RPM.
Thinking about the Heat/Energy generated by the combustion of fuel and how frequently it happens has an affect on what amount of fuel you want to enter the engine. At low RPM, there is a small amount of air blowing across the orifice in the venturi (throat) of the carb and at high RPM there is a large amount of air blowing across the orifice in the venturi. Getting enough fuel to keep the engine running at idle without fouling the spark plug, still snappy coming off idle and not stalling the engine is tough. Then you think about the changing demand for fuel as the engine revs. Midrange versus wide open throttle at Peak RPM. The higher the engine revs the more residual heat is built up in the combustion chamber and having a higher fuel to air ratio (rich) will help dissipate some of the heat away from the combustion chamber. That’s why engineers have so many variations on Carb Designs. Idle circuits, intermediate circuits and main circuits are all forms of delivering the right amount of fuel for the given air volume entering the engine.
Now you, the driver/tuner have to figure out what to adjust to get maximum performance out your engine without destroying it in the process! Its no wonder carburetor tuning has been likened to wizardry.
I know this thread is about Rich versus Lean, but at the heart of it I think is about how to adjust your Carb to deal with changing conditions and demands. Do I go Rich or do I go Lean and on which circuit do I make the change or do I change all of them? Knowing why you make certain changes and not others can help you get the most potential out of your Carb. I learned most of the following tuning Radio Controlled car and truck Engines. They ran on Nitro-Methane, but the principals are the same for tuning the carb on a kart.
In general a slightly lean Low Speed Needle/Jet setting will keep the engine snappy off throttle. Get too lean and engine will struggle to pull off idle. Get too rich and the fuel can coat the spark plug and cause misfires also making is struggle to pull off idle or even stall out. This is usually the first adjustment you make to the carb as it also affects the other adjustments. You can adjust this on the stand by repeatedly cracking open the throttle and listening to the engine note. If slightly rich, the engine will stumble then clear as the RPM’s increase. If too lean, the engine will struggle to rev up. It easier to start slightly rich and lean the low speed needle until you get a crisp response from the engine when you crack the throttle.
Midrange is where you will likely be at the ideal stoichiometric ratio for fuel and air mixture. The combustion is building heat, but not so much that it can’t naturally dissipate it by design. Too lean here and you’re leaving power on the table. Too rich here and the combustion cannot completely consume all of the available fuel. For a 2T that could also lead to Oil Fouling the spark plug and reduced performance. I haven’t seen many carbs with an adjustment for the mid-range setting as it is often influenced by a mix of the low speed and high speed fuel circuits. On some slide carbs there may be a jet associated with the midrange circuit that can be changed. Its hard to know which way to go without data (either Lambda or acceleration curve) to tell you if your changes make an improvement or not.
The High speed circuit is usually a touch on the rich side. Too rich will cause the engine to “four cycle” like TJ said. I think this happens because there is too much unburnt fuel leaving the combustion chamber and it continues to burn in the exhaust port. Sort of like a backfire on a 4T. Too lean will have an increase in combustion temperatures and likely lead to fuel vaporization prior to ignition. That has negative effects on how the fuel ignites and propagates through the camber. It can also lead to pre-combustion prior to the desired ignition point (pinging). Worst case you stick a piston or destroy the cylinder. A little lean mixture can be advantageous, but it comes at the risk of damage or increased wear. Unless you are the pinnacle of the sport and have multiple engines available, you should probably error on the side of caution. An example of when a slightly lean mixture could work is on a very tight course when you are not at peak RPM for more than a few seconds before backing out of throttle to brake for another corner. Slightly richer setting helps keep things cool on long flowing courses where you may spend most of the lap at WOT near peak RPM. One way to tune for a 2T using a EGT sensor is starting on the rich side and leaning the High Speed fuel circuit until your max temps peak and start to drop again at max RPM. Then richen the mixture back to where they peaked. As Aaron mentioned above, having the peak target temps is not always the fastest. In the case where his driver was faster with a richer fuel ratio on the high speed circuit was likely due to the fact that with no midrange adjustability, the engine was suffering from fuel starvation in the midrange more than it was benefiting from the correct fuel ratio at peak speed.
Another thing to consider for 2T’s is how much oil you are adding to your fuel for varying conditions. (RAD or Relative Air Density is a measure of the atmospheric density of the air relative to that of air at sea level as a percentage. It is a useful tool to know and track carb settings based on current conditions) For example in the summer months the Relative Air Density is lower than it is in the winter months. What does that mean? It means you have to lean the ratio of fuel to air during the summer to reach the desired stoichiometric ratio and thereby are also lowering the amount of oil going into the engine as compared to the winter months where you are richening fuel to air ratio and increasing the amount of oil going into the engine. Its not a lot, but for those that want to get peak performance, you can usually adjust your oil-fuel mix ratios slightly to account for this. Its another reason you are more likely to foul a spark plug in the winter and stick a piston in the summer.
Not sure if this adds to the topic or not. Hopefully it answers a few things for some of the new folks. Like all tuning in karting, if you are not seeing the desired results from a change, try going the other way. Meaning if leaning out the mix does not improve things, try richening it instead. For carbs that have external needle adjustments, you can usually tune the high speed needle on the fly by adjusting it leaner and leaner on the long straights until you do not see any more improvement in peak RPM, then richen it slightly to put it in the ideal range.