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14-20. MAINTENANCE.
Gyro filters should be re-
placed each 100 hours of engine operation. Relief
valve filters should be inspected each 100 hours, and
if dirty, cleaned with solvent and dried with com-
pressed air. They should be replaced at engine over-
haul intervals.
The vacuum pumps are designed to
operate without internal lubrication. Certain internal
parts of carbon compounds wear away at a minute,
controlled rate. Adequate precautions must be taken
to assure that oil, grease, thread lubricants, clean-
ing fluids or other foreign material cannot enter the
pumps. When blowing out lines and hoses, always
disconnect them at both ends.
When checking for
leaks, disconnect at both ends and plug one end.
Apply pressure to the opposite end and use a coat-
ing of soap and water to detect leaks. Pumps and
gyro instruments should be overhauled at engine
overhaul intervals.
14-21. RELIEF VALVE ADJUSTMENT. Since a
relief valve is used for each suction source, each
relief valve must be adjusted separately.
a. Start the rear engine.
b. Turn the vacuum test selector valve to either of
the two,gyro positions and check suction gage reading
at 1900 rpm.
c. Suction should indicate 4. 5 to 5. 00 inches of
mercury.
d. Using the trial-and-error method, adjust
knurled screw on rear relief valve to obtain the
desired suction gage reading. Do not adjust while
the engine is operating. Clockwise rotation of the
adjusting screw increases suction.
e. Start the front engine and shut down the rear en-
gine.
f.
Check the suction gage reading, with the vacuum
test selector valve still set at either gyro position,
at 1900 rpm.
g. Again, the suction gage should indicate 4.5 to
5.00 inches of mercury.
h. Adjust knurled screw on front relief valve to
obtain the desired suction gage reading.
This adjust-
ment may be made while the engine is running, since
the front relief valve is located inside the cabin.
i.
With both engines operating at 1900 rpm, turn
the vacuum test selector valve to the front source,
then to the rear source. Because of different line
lengths and other factors, readings at the sources
may not be the same, but both should be more than
the readings at either gyro position.
j.
Shut down both engines and check that the lock-
nuts at the knurled adjusting screws on the relief
valves are tight.
14-22.
REMOVAL AND INSTALLATION OF COM-
PONENTS.
Figure 14-4 shows details of the vacu-
um system and may be used as a guide for replace-
ment of parts. New lines and hoses should be thor-
oughly cleaned before installation, and adequate pre-
cautions should be taken to prevent any foreign ma-
terial from entering the system. Early type vacuum
pumps have a spring-steel band around the neck of
the pump which mustbe removed before the pump
mounting bolts can be removed. Later type vacuum
pumps are not equipped with this band.
14-23. ENGINE INDICATORS.
14-24. DUAL TACHOMETER.
The dual tachometer
is an electronic instrument which converts the elec-
trical current from pick-up coils to movement of the
pointers along scales indicating engine rpm. Each
pick-up coil screws into the bottom side of the right
magneto on each engine, close to the rotating magnet,
so that current may be induced in the pick-up coil by
the rotating magnet.
14-25. TROUBLE SHOOTING THE DUAL TACHOMETER.
PROBABLE CAUSE
ISOLATION PROCEDURE
NO INDICATION FOR EITHER ENGINE.
Circuit breaker out.
Defective circuit breaker.
Defective "hot" wire to tach-
ometer or defective ground
wire.
Defective tachometer.
Check visually.
Check continuity.
Check continuity through wires.
Substitute known-good tach-
ometer and check operation.
Reset circuit breaker. If it pops
out again, determine cause and
correct.
Replace circuit breaker.
Repair wiring.
Replace tachometer.
REMEDY
14-13
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