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Home> Continental & Lycoming TroubleShoot
Idle mixture adjustment on Marvel Schebler (Precision) carburetors1. Engine should be warmed up. Make sure engine has proper timing, compression, and is in otherwise good condition. The carburetor must also be in good condition.2. Face the aircraft into the wind. 3. At 650 rpm (or minimum rpm) you should get no more than 25 to 50 rpm rise in rpm when slowly moving mixture control toward idle cut-off. If you get more than 50 rpm rise then the carburetor idle mixture is rich and needs to be leaned. If you get no or less than 25 rpm rise then the carburetor idle mixture is too lean. Note: if the idle mixture is set too lean the engine will stumble (lean cut-out) when the throttle is advanced. 4. The idle mixture adjustment is a large knurled screw (or small slotted knob) on the throttle casting, high on the carburetor. On MA-3 and MA-4 series carburetors the slotted knob has arrows on it pointing to 'R' (rich) and 'L' (lean). Turn this screw in the desired direction. Adjusting idle mixture may have an effect on engine idle speed. You may have to adjust idle speed. The idle speed adjustment is a set-screw on the low-rpm stop at the throttle arm on the carburetor. Adjusting the screw increases or decreases throttle travel at the low rpm stop.
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