Engine Control
TTP
|
B1-L1
|
ATA76
|
Cessna
172
|
B2-L1
|
General - mixture control is the push-pull type that incorporates a threaded vernier mechanism for fine adjustment. The ball bearing rod end on the mixture control is secured to the engine with a predrilled AN bolt, a castellated nut and a cotter pin.
NOTE: Steel AN bolts with an undrilled shank are identified with an 'A' suffix (AN3-6A). A steel bolt of the same size, with the shank drilled for castellated nut and cotter pin is identified as AN3-6. Aluminum bolts and undrilled bolts must not be used in this application.
When adjusting the fuel mixture control, it is important to check that fuel mixture control slides smoothly throughout its full range of travel, that it adjusts through its full vernier range and the mixture arm operates through its full arc of travel. Do not lubricate fuel mixture control. If excessive binding is noticed, replace fuel mixture control.
C. Whenever engine controls are being disconnected, pay particular attention to the exact position, size and number of attaching parts as noted when connecting controls.
761001 Engine Controls and Linkage - Examine the general condition and freedom of movement through the full range. Complete a check for the proper travel, security of attachment, and for evidence of wear. Complete a check of the friction lock and vernier
adjustment for proper operation. Complete a check to make sure the throttle, fuel mixture,
and propeller governor arms operate through their full arc of travel. The maximum linear freeplay is 0.050 inch.
Fuel Mixture Control
Remove Fuel Mixture Control
(1) Remove engine cowling.
(2) Remove cotter pin, nut, bolt and washers securing mixture control rod end to throttle body mixture arm.
(3) Remove clamp securing fuel mixture control to mixture control bracket.
(4) Remove fuel mixture control retaining nut and washer from forward side of firewall.
(5) In the cockpit/cabin area, remove mixture control retaining nut and washer from forward side of instrument panel.
(6) Carefully pull mixture control through firewall and instrument panel, and remove from airplane.
B. Install Fuel Mixture Control.
NOTE: When installing mixture control ensure that control is routed exactly as previously installed.Ensure that no binding or preloading occurs from a too small bend radius.
(1) In the cabin/cockpit area, carefully route fuel mixture control through instrument panel, and then place washer and retaining nut over fuel mixture control rod end.
(2) Route fuel mixture control through firewall.
(3) Secure fuel mixture control in instrument panel by tightening retaining nut against washer and instrument panel.
NOTE: To prevent damage to the instrument panel finish and markings, ensure the control housing does not rotate against the instrument panel during installation.
(4) In the engine compartment, place washer and retaining nut over fuel mixture control rod end and secure against firewall.
(5) Attach mixture control rod end to throttle body mixture arm with bolt, washers, nut and cotter pin.
(6) Secure fuel mixture control to mixture control bracket with clamp.
(7) Install Engine Cowling.
Fuel Mixture Control Adjustment/Test
A. Check Fuel Mixture Control.
(1) Push fuel mixture control full in and verify that mixture arm on throttle body is fully open (rich).
(2) Pull fuel mixture control full out and verify that mixture arm on throttle body is fully closed (lean).
(3) Do a check to make sure that the fuel mixture control has no less than 0.12-inch (3.18 mm) and no more than 0.25-inch (6.35 mm) cushion at each stop.
(4) Work fuel mixture control in and out several times to check for binding.
B. Adjust Fuel Mixture Control.
(1) Disconnect fuel mixture control rod end from throttle body.
(2) Loosen jam nut and adjust rod end to obtain desired setting. The witness hole in the rod end must be covered with the mixture cable threads.
(3) Tighten jam nut.
(4) Connect rod end to throttle body. If necessary, you can reposition the mixture control housing in the mixture control bracket clamp.
4. Fuel Mixture Control Inspection/Check
A. Inspect Fuel Mixture Control.
(1) The mixture control attachment to the throttle body should be inspected in accordance with time limits established in Chapter 5, Inspection Time Limits . Check bolt, castellated nut, cotter pin and rod end for security and condition. The witness hole in the rod end must be covered with the mixture cable threads. Check that fuel mixture control slides smoothly throughout its full range of travel, that it adjusts through its full vernier range and the mixture arm operates through its full arc of travel.
Precision Airmotive Service Letter SIL RS-67.
(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 enrichen 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 enrichen 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.
NOTE: Steel AN bolts with an undrilled shank are identified with an 'A' suffix (AN3-6A). A steel bolt of the same size, with the shank drilled for castellated nut and cotter pin is identified as AN3-6. Aluminum bolts and undrilled bolts must not be used in this application.
When adjusting the fuel mixture control, it is important to check that fuel mixture control slides smoothly throughout its full range of travel, that it adjusts through its full vernier range and the mixture arm operates through its full arc of travel. Do not lubricate fuel mixture control. If excessive binding is noticed, replace fuel mixture control.
C. Whenever engine controls are being disconnected, pay particular attention to the exact position, size and number of attaching parts as noted when connecting controls.
761001 Engine Controls and Linkage - Examine the general condition and freedom of movement through the full range. Complete a check for the proper travel, security of attachment, and for evidence of wear. Complete a check of the friction lock and vernier
adjustment for proper operation. Complete a check to make sure the throttle, fuel mixture,
and propeller governor arms operate through their full arc of travel. The maximum linear freeplay is 0.050 inch.
Fuel Mixture Control
Remove Fuel Mixture Control
(1) Remove engine cowling.
(2) Remove cotter pin, nut, bolt and washers securing mixture control rod end to throttle body mixture arm.
(3) Remove clamp securing fuel mixture control to mixture control bracket.
(4) Remove fuel mixture control retaining nut and washer from forward side of firewall.
(5) In the cockpit/cabin area, remove mixture control retaining nut and washer from forward side of instrument panel.
(6) Carefully pull mixture control through firewall and instrument panel, and remove from airplane.
B. Install Fuel Mixture Control.
NOTE: When installing mixture control ensure that control is routed exactly as previously installed.Ensure that no binding or preloading occurs from a too small bend radius.
(1) In the cabin/cockpit area, carefully route fuel mixture control through instrument panel, and then place washer and retaining nut over fuel mixture control rod end.
(2) Route fuel mixture control through firewall.
(3) Secure fuel mixture control in instrument panel by tightening retaining nut against washer and instrument panel.
NOTE: To prevent damage to the instrument panel finish and markings, ensure the control housing does not rotate against the instrument panel during installation.
(4) In the engine compartment, place washer and retaining nut over fuel mixture control rod end and secure against firewall.
(5) Attach mixture control rod end to throttle body mixture arm with bolt, washers, nut and cotter pin.
(6) Secure fuel mixture control to mixture control bracket with clamp.
(7) Install Engine Cowling.
Fuel Mixture Control Adjustment/Test
A. Check Fuel Mixture Control.
(1) Push fuel mixture control full in and verify that mixture arm on throttle body is fully open (rich).
(2) Pull fuel mixture control full out and verify that mixture arm on throttle body is fully closed (lean).
(3) Do a check to make sure that the fuel mixture control has no less than 0.12-inch (3.18 mm) and no more than 0.25-inch (6.35 mm) cushion at each stop.
(4) Work fuel mixture control in and out several times to check for binding.
B. Adjust Fuel Mixture Control.
(1) Disconnect fuel mixture control rod end from throttle body.
(2) Loosen jam nut and adjust rod end to obtain desired setting. The witness hole in the rod end must be covered with the mixture cable threads.
(3) Tighten jam nut.
(4) Connect rod end to throttle body. If necessary, you can reposition the mixture control housing in the mixture control bracket clamp.
4. Fuel Mixture Control Inspection/Check
A. Inspect Fuel Mixture Control.
(1) The mixture control attachment to the throttle body should be inspected in accordance with time limits established in Chapter 5, Inspection Time Limits . Check bolt, castellated nut, cotter pin and rod end for security and condition. The witness hole in the rod end must be covered with the mixture cable threads. Check that fuel mixture control slides smoothly throughout its full range of travel, that it adjusts through its full vernier range and the mixture arm operates through its full arc of travel.
Precision Airmotive Service Letter SIL RS-67.
(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 enrichen 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 enrichen 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.
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