Nearly every all-terrain vehicle produced since the 1980s relies on an internal combustion engine as a power source. In order to operate effectively, the engine requires a certain ratio of air and fuel to create the combustive force it needs. A rich operating condition is caused by the addition of more fuel into the mixture. While it can be slightly beneficial in certain circumstances, a rich condition can cause problems for your ATV.

Understanding Air and Fuel Mixtures

• Internal combustion engines require two things to generate power: air and fuel, in the form of liquid gasoline. Both are mixed inside of the ATV’s carburetor, where small nozzles, called jets, spray a measured amount of gasoline into the air that is being pulled through the carburetor’s inlet and into the engine. The fuel suspended in the mixture is atomized, or reduced into tiny droplets, that help the ignition process within the engine. While the fuel itself is flammable, the presence of oxygen in the air enhances the fuel’s ability to ignite and combust. Most carburetors are preset to allow enough fuel into the incoming air stream to provide the most efficient mixture possible.

The Effects of a Rich Operating Condition

• The addition of more fuel into the air stream, either through deliberate modifications or through a lack of maintenance, will manifest itself as a loss of power or surging. This is caused by the fuel’s inability to effectively ignite without the presence of air. For example, a campfire can be stoked by gently blowing air over it. That same fire would extinguish if it were placed in a complete vacuum, because oxygen would disappear. The same occurs within the engine when oxygen levels are lowered or fuel levels increased. Visually, a rich running condition will often leave a wet, black appearance from unburnt fuel on the ATV’s spark plug. As a side effect, the ATV’s fuel economy will decrease, because it uses more fuel than normal.

Possible Causes

• A completely stock ATV can encounter a rich condition for several reasons. First, a dirty air filter will reduce the amount air entering into the engine. Likewise, a damaged or improperly set carburetor float will allow more fuel into the carburetor, allowing the excess to be pulled into the air and fuel mixture. If the ATV is operated at higher elevations, the decrease in ambient oxygen will also cause a rich condition. However, the most likely cause of a rich condition is the alteration of the carburetor’s fuel settings through the replacement of its fuel jets. Most ATV owners replace the jets to “tune” the machine for better performance, which can be done providing that the air intake and exhaust systems are modified to suit.

Fixing the Problem

• Finding a solution to a rich operating condition depends largely on any modifications made to the machine and where it is being ridden. A completely bone-stock ATV could benefit from a cleaning — or replacement — of the air filter to ensure that enough air is reaching the engine. Likewise, checking the carburetor’s float level can prevent the carburetor from over-filling. However, if you are using the ATV at high elevations, reducing the carburetor jets by one full size may help restore power. Finally, modified ATVs must also have a free-flowing air filter and exhaust installed to benefit from a larger fuel jet. Without these additional items, the carburetor will simply be using more fuel and making less power with the stock air flow settings.

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