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A modern four stroke off-road motorcycle is designed to run on a mixture of fuel (gasoline) and air (oxygen) and will perform its best at an air to fuel ratio of 12:1 to 14:1 depending upon a few variable factors, including the type of fuel.

When the mixture gets out of this range it can cause a noticeable loss of power, poor starting, damage to internal components, such as the inlet and exhaust valves, and it can cause the engine to overheat.

When the throttle of an internal combustion engine is opened rapidly the engine can quickly suck a large volume of air through the intake. Fuel injection engines monitor this increase in air and pump in enough fuel to keep the mixture within a very precise range. Unfortunately, a carburetor as normally seen on the competition off-road motorcycle relies on the jets and an accelerator pump to compensate; this small pump literally sprays a small jet of fuel into the engine to richen the mixture to an acceptable level. If the amount of fuel isn’t increased to match the volume of the air, the bike can hesitate, or “bog”.

It’s very common for the jetting to be overly rich so the engine doesn’t bog and very common to need carburetor adjustments to compensate for altitude, temperature and even fuel. The best way is to regulate the jetting for best performance and then adjust the accelerator pump to match the air flow.

An excellent way to adjust the accelerator pump correctly is by changing the leak jet located in the bottom of the float bowl. This jet allows excess fuel back into the bowl so the smaller the jet, the greater the flow of fuel into the engine. Unfortunately, changing the leak jet can be time consuming, and purchasing a range of jets, expensive. After you install this new kit, the leak jet will be replaced with an adjustable leak circuit so you can get the engine “dialed in” no matter where you are.

Adjustable Leak Jet (ALJ)
all FCR MX carbs (00-008)