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Scoot F.A.Q.


Q. What does it mean if my scooter is running rich or lean?

If you want the simplest explanation, here it is :

Rich = Too Much Fuel / Not Enough Air
Lean = Not Enough Fuel / Too Much Air


If you'd like a little more info, read on. Rich and lean refer to air to fuel ratios (air/fuel ratio, AFR). An air/fuel ratio is expressed as a number like 10:1. An air/fuel ratio of 10:1 would mean that there are 10 parts air to 1 part fuel present in a mixture, measured by mass (kilograms, pounds). The fuel mass is always 1 and the air mass number changes for our purposes. For example, 10:1, 12:1, or 16:1. The higher the first number is, the more air is present, or the more lean the mixture is. That's all nice to know, but just stating some ratio of air and fuel mass doesn't tell us anything especially useful without more information.

Fuels have a stoichiometric ratio which tells us the air/fuel ratio necessary for complete combustion. Complete combustion is when a hydrocarbon fuel (C8H18 for example) and oxygen (O2) in the air react in a way that only carbon dioxide (CO2) and water (H2O) are produced. If more air is present than needed (lean mixture), combustion can still be complete but there will be leftover excess air. Lean mixture ratios may burn hot and cause more nitrogen oxide (NOx) emissions. If not enough air is in the mixture (rich mixture), more harmful emissions such as carbon (C) in the form of black soot or smoke or carbon monoxide (CO) and even unburned fuel may be present in emissions. From an emissions standpoint, stoichiometric ratios would be the best, but that's not the end of the story.

Different fuels have different stoichiometric ratios, but most of us will be using gasoline which has a stoichiometric ratio of 14.7:1 so I'll use that for the remainder of this explanation. Based on the above information, you may think that your goal as a scooter tuner is to tune your gasoline engine so that it's always supplied with a stoichiometric 14.7:1 air/fuel ratio... but you would be wrong. Maximum power is usually made somewhere between 12:1 to 13.5:1, though it can vary depending on the exact setup. The fuel and air delivered to the the combustion chamber is never completely uniformly mixed, so there will be richer and leaner areas rather than one perfect homogeneous stoichiometric mixture available to be used for complete combustion. The goal is to use up as much of the oxygen present in the cylinder, so more fuel is needed to ensure that oxygen in the cylinder has fuel nearby to react with. This is why maximum power air/fuel ratios are on the rich side of the stoichiometric ratio. Rich mixtures also keep the engine cooler, which can be especially helpful on air-cooled, two-stroke, and highly tuned applications.

Best fuel economy usually comes from air/fuel ratios around 16:1. Such lean ratios are only suitable for light load, low throttle conditions, and may not be suitable at all for air cooled engines. Two-strokes especially need to remain at richer mixture ratios.

Most scooter tuners don't even know their air/fuel ratio, because measurement requires an air/fuel ratio gauge and a wideband oxygen sensor plumbed into the exhaust. These tend to be somewhat expensive, require welding a bung into the exhaust, and tuning can be done without the use of a wideband so they are looked at as somewhat of a luxury. Two-strokes are also known to have problems fouling sensors and with inaccurate readings.

O2 Sensor In 139QMB Exhaust
Wideband Air/Fuel Gauge


More common ways to determine proper air/fuel ratio are :
  • Trial And Error Tuning : Looking for the best results (acceleration, speed, RPM, throttle response) while making changes to the tune via jetting or adjustments.

  • Spark Plug Reading : Examining used spark plugs for signs of rich and lean conditions.

  • Observation : Taking note of sounds, symptoms, or other gauges that provide insight into mixture properties.

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