Hi, Anonymous you need to make sure you have no intake leaks by spraying some Brake Kleen, Contact Cleaner, Starting Fluid, etc. around the intake manifold and seals, if the RPM's go up or down significantly then you have a leak that needs to be fixed before you can continue with the carburetor adjustment.
The engine should be at operating temperature with the choke off to set the idle between 950-1050 rpm. Turn the idle skrew clockwise to increase the idle speed and counterclockwise to decrease the idle speed. If you have access to your air-fuel mixture skrew you may turn it as well, if not there is a video below on how to gain access, make sure you start with clean or new spark plugs. Gently turn the air-fuel mixture skrew clockwise until it starts to bottom out "STOP" do not over tighten as it will damage the taper on the end, now back the skrew out 1-1/2 full turns to establish a starting point start the engine and check the idle rpm's and turn the idle skrew accordingly for 950-1050 rpm if necessary, turn the air-fuel mixture skrew 1/4 turn at a time and let the engine settle for about 10 seconds, turn the skrew clockwise for a leaner mixture and counterclockwise for a richer mixture, you are seeking the highest rpm your air-fuel mixture skrew can acquire without going past 3 full turns from the bottom then resetting the idle skrew back down to 950-1050 rpm If you can not detect any rpm change and can not stall the engine by turning the air-fuel mixture skrew all the way in then your pilot/low-speed jet is too big and needs to be replaced with the next size smaller. Final adjustment should be made with a clean air filter mounted to the carburetor.
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Adjust Engine Idle Ninja 250