If you've never heard the term "air jetting" before, don't worry.  Tuners have been doing it for years, but until recently there had been no way for the rest of us to practice this black art.
     In snowmobiling, air jetting has taken the form of tuners to modifying factory airboxes.  One common mod is simply removing the airbox and replacing it with pod filters.  This method usually requires significant other modifications in order to work properly and it also requires relocation of parts that are often mounted to the stock airbox.  Tuners have also gutted airboxes to completely remove all restrictions from the airbox.  Once these changes are made there is no going back, and changes like this have often resulted in the opposite effect: A decrease in performance.
     There are legitimate reasons for wanting to modify and airbox.   Added air flow through the box could very well increase engine power.  Tuners know airboxes can be "timed", to resonate very strongly in step with the engine's intake pulses.  If done properly, the engine takes air from the box only during the high pressure point in its cycle, resulting in a gain in engine torque.
     Another reason for airbox modifications is to improve performance for higher altitudes where air has very low density.  Increased airflow can address this problem,. minimizing the need to disassemble the carburetor and make jetting changes.
     Lately a new patent pending air control valve called the Total Performance Intake Valve (TPI Valve) from Thunder Products has come on the market that takes some voodoo out of air tuning.  The TPI Valve allows more control of the amount of air entering the airbox by providing an add-on valve(s) that can be adjusted to flow more or less air simply by twisting a control from closed to open, or anywhere in-between.
     The valve is 1 1/2 inch tall and fits into a 1 1/2 inch hole drilled into the airbox with a hole saw.  Any number of valves can be used to get the desired effect - Thunder Products recommends one TPI Valve per 200cc of engine displacement.  The TPI valve can be installed in a neutral position, equal distance from the carburetors to allow the jetting to be adjusted simultaneously.  By mounting the valve close to a carburetor, the added air will favor that carburetor and can be used to balance the jetting from one cylinder to the other.
     The term "air jetting" refers to adjusting the air flow into the carburetors to adjust the air/fuel mixture rather than adjusting the fuel flow.  Instead of using a smaller jet size to adjust a rich condition, a tuner can easily increase the air flow, opening the valves and running the engine to take spark plug readings, repeating the process until spark plug electrodes indicate proper engine jetting.  Thunder Products dyno test results indicate a four horsepower gain in a stock 700cc triple using only the TPI Valve to adjust the air/fuel mixture. 
     TPI Valves can make experimentation quick and easy, allowing use of the original airbox configuration, and open configuration, or anywhere in between.  It is also done without making permanent changes to the airbox.  A "low boy" valve is being developed for tighter fitting applications.
     For more information about "air jetting" and the TPI Valve, contact Thunder Products at 320-597-2700 or go to www.thunderproducts.com.