How to Tune a Dirt Bike With a UNI Air Filter
One of the first components to be replaced by the serious off-road rider or motocross racer is the air filter. UNI, a manufacturer of high-performance air filters, has been equipping aspiring motocross heroes and serious trail riders since 1971. However, simply installing a UNI filter without any other high-performance components will actually have a negative effect on your dirt bike's performance. To really "tune" your dirt bike, you will need to add a few more components to balance your dirt bike's air-to-fuel ratio.
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Instructions
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Tuning
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1
Install a UNI high-flow air filter to replace the stock air filter. This allows are greater volume of air to enter into the motor.
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2
Install larger jets in the carburetor to match the increase in air volume. When re-jetted properly, the carburetor will create a stronger air/fuel mixture that will increase power.
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3
Replace the factory-equipped exhaust system with an aftermarket exhaust to help expel spent gases as directly as possible. Most aftermarket pipes provide a straight, unrestricted flow that helps pull in the next charge of fresh air and fuel as the exhaust gases are released.
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4
Start your dirt bike, listening to the motor for pinging or an unusually high idle, as these are signs that the fuel mixture is too lean. If the bike has trouble starting or will not stay running, it is still running too lean and will need a larger set of jets.
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5
Test ride your dirt bike, looking for any signs of engine surging or pinging. Testing for a lean mixture can also be done by revving the motor then letting off the throttle. If the motor hangs at a higher idle then tapers off, it is still running lean. If any of these symptoms are present, install a larger set of jets.
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6
Remove the carburetor and replace the jets with a larger series of jets, if needed, to balance the air/fuel ratio. Take the bike out for another test ride, looking again for symptoms of a lean condition. Repeat as necessary.
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7
Go for a long ride.
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Tips & Warnings
Although the stock filter is adequate for an unmodified dirt bike, a UNI air filter allows a denser volume of air to pass through the filter media to create a stronger burn, or combustion, within the motor. Next, the carburetor's jets, which allow fuel to mix with the incoming air before it enters the motor is set to specific ratio that is determined by the stock air filter and carburetor jetting to provide the all-around performance. With the increase in air volume afforded by the UNI filter, the air-to-fuel ratio is thrown off-balance and creates a lean condition, meaning that there is less combustible fuel to make power with. Installing a larger jet allows more fuel to enter the mixture, restoring the air to fuel ratio and providing an increase in power. The resulting increase in air and fuel creates a larger amount of spent gases that must exit as quickly as possible to allow a fresh charge to enter the motor. The stock exhaust tends to be a bit restrictive, due to sound emissions requirement, forcing the exhaust gases to travel through a series of chambers and baffles. Performance exhaust systems eliminate a large portion of the twists and bends inherent to stock setups by providing a direct flow which increases power as a result.
Take your time when replacing the carburetor's jets. Besides being a bit complex to the uninitiated, you may need to re-jet the carburetor several times using increasing larger jet sizes to fine-tune the power band of your dirt bike.
If you do not possess the tools or knowledge to complete this project, seek the help of a qualified technician.
Use only fresh fuel when riding, this is especially true for dirt bikes that are stored for long periods of time. Gasoline has a tendency to break down over time and will cause poor performance.
Properly oil the air filter according to UNI's instructions to prevent dirt and contaminants to enter the motor.
