Thursday, July 19, 2012

Cessna 152 Magneto Check


MAGNETO CHECK.
NEVER ADVANCE TIMING BEYOND SPECIFICATIONS IN ORDER TO REDUCE RPM
DROP. Too much importance is being attached to RPM drop in single ignition. RPM drop on single ignition is a natural characteristic of dual ignition design. The purpose of the following magneto check is to determine that all cylinders are firing. If all cylinders are not firing, the engine will run extremely rough and cause for investigation will be quite apparent. The amount of RPM drop is not necessarily significant and will be influenced by ambient air temperature, humidity, airport altitude, etc. In fact, absence of RPM drop should be cause for suspicion that the magneto timing has been bumped up and is set in advance of the setting specified. Magneto checks should be performed on a comparative basis between individual right and left magneto performance.
a. Start and run engine until the oil and cylinder head temperatures are in normal operating ranges.
b. Advance engine speed to 1700 RPM.
c. Turn the ignition switch to the "R" position and note the RPM drop. then return the switch to the "BOTH" position to clear the opposite set of plugs.
d. Turn the switch to the "L" position and note the RPM drop. then return the switch to the "BOTH" position.
e. The RPM drop should not exceed 125 RPM on either magneto or show greater than 50 RPM differential between magnetos. A smooth RPM drop-off past normal is usually a sign of a too lean or too rich mixture. A sharp RPM drop-off past normal is usually a sign of a fouled plug, a defective harness lead or out of time. If there is doubt concerning operation of the ignition system. RPM checks at a leaner mixture setting or a higher engine speeds will usually confirm whether a deficiency exists
d. Tighten friction lock, being careful not to change position of the throttle.
e. Move throttle arm on the carburetor to full open, adjust rod end at end of throttle control to fit, and connect to arm on carburetor.
f. Release friction lock and check full travel of arm on carburetor. If further adjustmer is required, make all adjustments at the carburetor end of control. DO NOT change jam nut setting.
g. Tighten rod end locknuts at carburetor end of control. Be sure to maintain sufficient thread engagement between rod end and control.

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