Arctic Cat 900 twin engines fitted with carbs come with Mikuni rack style flatslide (TM-40mm) carburetors. These carbs have an airscrew and a fuel screw, both providing adjustability to the low speed circuits. The air screw is located to the left side of the mouth of the carb, and the fuel screw is adjusted with the white knob on the right side of the carb.
Many tuners do not understand when to use each of these adjustments. The airscrew affects up to about 3/8 throttle, with an adjustment range of 1/16 turn to 3 turns. The fuel screw only affects up to about 1/8 throttle, and has an adjustment range from 1/4 turn to 5 turns.
If you experience poor throttle response between 1/8 and 3/8 throttle you should adjust the airscrews for optimal throttle response. Some tuners will also encounter needle adjustment issue at these throttle positions where the needle position theyâ€™re running is too lean, but the next richer clip position is too rich. The fix here is to install a 0.020â€ shim under the e-clip, allowing you to effectively adjust the needle position in 1/2 position increments (each e-clip position changes needle position by 0.040â€). If the e-clip is in the third position with a 0.020â€ shim under it, you basically have a 3.5 position.