General Info about racing filters installation
It is very important to know that, when you are about to replace your OEM air filter with a high flow (so called racing) one, you will probably have to tune your carb(s) properly in order to work fine with the new performance part you have installed.Carb tuning for this purpose is also known as rejetting.This process we'll try to explain in this short guide.
Factory set carbs and technical specifications
Carbs control the amount of fuel that enters the engine.When the bike or car comes out from the factory the carbs are adjusted this way that a very specific amount of fuel is released.This amount is calculated by the engine designers who have carefully considered some certain factors such as expected horsepower,fuel consumption,fumes restrictions (eg EURO2 legislation) and other technical specifications set by some countries or EU (such as the 90klm/h japanese restriction or the 100 hp maximum power in some EU countries for motorbikes).
In order to achieve all that the factory technicians use parts that might limit the overall horsepower but would serve their purpose to keep the production cost low and make the final product be in accordance to any law or market requirements.Such parts are usually the air filters,the exhausts and the cams which are the first to replace when we are looking for higher performance.
The carbs are set to release as much fuel as needed when the engine is working with the OEM parts.When one of these parts is replaced with a performance aftermarket one then the thin balance is disturbed and more actions are required to restore it and get more horse power out of the engine.
Proper carb tuning & racing filters
So in order to make our engine run perfectly fine and get more horsepower from a high flow filter we have to tune our carbs.That means that we have to open the carb and replace the main jet with a new one more suitable for the air supply the new filter provides.For that reason this tuning process is also called rejetting.
The truth is that the OEM filter in fact allows very little air to go through.For that reason the main jets are quite small.When you replace the OEM filter the engine needs more fuel so you have to install a bigger jet.This jet should be about 10 to 15 numbers bigger size (eg. a number 80 jet should be replaced with a number 90 to 95 one).Usually the jets have their size printed on them.
Unfortunately there is no standard rule on how to perfectly tune your carb.The suggestion I give you above is based on my own experience from races and nothing more.The truth is that perfect jetting is a trial and error process, especially when you are looking for perfection.But that is something that only bothers a race rider/driver who is after a split second better lap time and not the average one.
When you have upjetted your carb you should immediately test ride your bike and see how it reacts to the changes you've made.if you still feel it heavy at higher RPM you might need to upjet it a couple of sizes more.If though you feel it heavy at lower rpm and unwilling to rev then you have to down jet it a couple of sizes(that's rather rare if you follow my advice 2 paragraphs above).
Rejetting isn't always necessary as the carb might be already rich in fuel (for many different reasons) so the engine might work fine with the new filter without further changes.
If you have any questions about filters,we,at BK Racing Parts,are always willing to help.Just drop us a line.