Cessna 172R Idle Speed and Mixture Adjustment

(1) Make sure that the alternate air door is in the closed position during this adjustment.
(2) Operate the engine until the oil temperature increases to 150°F (65°C).
NOTE: It may not be possible to get an oil temperature of 150°F (65°C) at cooler ambient temperatures. In that condition, it will be necessary to set the idle speed and mixture at a lower temperature.
(3) With the mixture control in the full rich position, set the idle speed to 675 RPM, +25 or -25 RPM.
(4) Advance the throttle to approximately 1800 RPM and immediately return it to idle. Idle speed should be approximately the same as set above.
(5) Adjust the fuel mixture control by rotating the knob counterclockwise, toward lean, quickly for approximately one inch, then very slowly until the peak RPM is obtained and the engine speed starts to drop off.
(a) When the engine speed first starts to increase, you will see a slight rise in RPM.
NOTE: Do not mistake this as the total RPM rise.
(b) Continue the slow rotation movement of the mixture control until you will see or sense a drop in the engine RPM.
(c) The maximum RPM before the drop in engine RPM is the total RPM rise which indicates the mixture strength at the engine idle speed.
(6) If the rise is less than 10 RPM it is necessary to enriching the fuel mixture.
(7) If the rise is more than 50 RPM it is necessary to lean the fuel mixture.
NOTE: To aid in the adjustment of the fuel mixture, the clevis on the fuel servo has an L (lean) and R (rich) stamp on it to indicate the direction that the thumb wheel should be moved to enrich the fuel mixture and increase the RPM rise. Turn the thumb wheel in the opposite direction you will lean the fuel mixture and decrease the RPM rise.
(8) Adjust the thumb wheel to set a rise 10 to 50 RPM.
(9) If the adjustment thumb wheel bottoms out on the blocks, center it as follows:
(a) Measure the distance between the two blocks.
(b) Disconnect the spring from the linkage pin.
 (c) Remove the cotter pin, linkage pin, wave washer, and flat washer.
(d) Turn the block and adjustment screw until the adjusting thumb wheel is centered.
(e) The distance between the blocks should measure the same as above.
(f) Install the linkage pin, flat washer, wave washer, cotter pin, and spring.
(10) After each adjustment is made, the engine speed should be increased to approximately 1800 RPM and held for approximately 10 to 15 seconds to clean the spark plugs and clear the cylinders of excess fuel.
(11) Put throttle in idle position
(12) Repeat the procedure until you get the desired RPM rate change at idle.
NOTE: If the mixture was excessively rich or lean when this procedure was started the engine speed will require readjustment as the fuel mixture is adjusted to the desired value. Set the idle speed to the specified RPM after the mixture has been set to get the 10 to 50 RPM rise a lean condition.
(13) Operate engine to full throttle and back to idle to make sure that the setting has not changed.
NOTE: Small changes in the idle speed and RPM are permitted. Find the cause of any large variations in RPM.    
Reference   : Precision Airmotive Service Letter SIL RS-67/ Cessna MM Rev.18
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