An unusually high idle speed can be both distracting and dangerous to any Ninja 250R rider, drawing the rider's attention away from the road. While a high idle can sometimes be attributed to a low fuel level, most high-idle situations appear in one of three forms: an idle speed slightly higher than 1,300rpm, a high rpm that will not change regardless of changes to the idle speed, or as a hanging idle when the throttle is closed.
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Inadequate Fuel Level
Check that your Ninja 250R's gas tank is filled with fresh gasoline and that the tank's fuel valve is set to the "On" or "Reserve" position. Carburetted Ninja 250R models, especially earlier models, have a tendency to increase the idle speed as fuel levels approach minimum.
Basic Idle Adjustment
Improper idle setting is the simplest, and most common, cause of high idle on a Ninja 250R. Open the choke completely, then start the motor. All Ninja 250Rs require a full three to five minutes of choke before the motor warms up to its operating temperature. During the warm-up, the motor will rev to 5,000rpm; this is normal. Do not rush through the warm-up phase, it is critical to attaining and maintaining the right idle speed. Once the motor has warmed, close the choke and observe the motorcycle's tachometer. Ideally, your Ninja should settle into a 1,300rpm idle. If the idle is higher than 1,300rpm, adjust the idle speed using the idle adjustment knob below the left side of the gas tank until the idle speed decreases to 1,300rpm.
Idle Speed Won't Decrease
A sudden increase in the engine's idle speed from the moment it is started is a symptom of an over-tightened throttle cable. The tension on the cable prevents the carburettors' throttle valves from closing completely, creating a high idle speed between 2,000 to 5,000rpm. Twist the throttle grip slightly. The grip should move at least 2 to 3 millimetres before it begins to pull the throttle cable, felt as a slight resistance. If you feel resistance immediately when twisting the throttle grip, loosen the throttle cable's locknut with a 10mm wrench. Turn the threaded adjuster screw at the cable's base clockwise by hand to loosen the cable. Leave your motorcycle running and listen to the idle speed as you loosen the cable. The idle should decrease steadily. Once the idle speed has levelled out to 1,500 to 1,300rpm, tighten the cable's locknut against the throttle housing with your 10mm wrench. Reset the base idle speed to 1,300rpm, if needed.
Idle Hangs at Higher RPM Then Drops to 1,300 RPM
A hanging idle between the 5,000 to 3,000rpm range that occurs when you're closing the throttle is normally caused by a damaged carburettor flange allowing air into the motor. You will need to remove the motorcycle's seat and side covers, before unscrewing the bolts at the base of the gas tank with a 10mm socket and a socket wrench. Prop the tank up with a block of wood to access the carburettors and their flanges. Visually inspect the flanges for signs of damage. If the clamps are tight and the flanges look intact, start the motor and complete its full warm-up procedure. Spray a small amount of a rubber-safe penetrating oil around the carburettors' flanges, one at a time, and listen for changes in the engine's idle speed. A slight drop in the idle is an indication that the flange has been compromised and will need to be replaced.
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