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Chapter 13 Preventive Maintenance and Troubleshooting
can be eliminated as a potential problem. In a matter
of seconds, you have determined that your trouble-
shooting efforts should be concentrated in the areas
of carburetion and ignition.
The engine’s owner can also provide assis-
tance with your troubleshooting efforts. Ask a few
questions about the engine’s performance before
it stopped. Relate the answers to the operating
requirements. For example, if an engine runs for
30-45 minutes and then stalls, you should ask if it
restarts immediately after it stops. If the answer
is yes, the problem is probably an ignition compo-
nent that is intermittently experiencing heat-related
breakdown. If the engine must cool before it will
restart, vapor lock or sticking valves are possible
problems. If a metallic snap is evident in the engine
during the cooling period, the valves are likely to
be the problem. The time you spend to ask a few
pertinent questions can save a lot of time in the long
run by eliminating additional problem possibilities.
Checking RPM
When servicing small engines, it is often nec-
essary to test or set maximum idle rpm or gover-
nor rpm. One way to do this is by using a device
that converts engine vibration from power pulses
to rpm. See Figure 13-25. To use this tool, place the
base of the instrument against the running engine.
Using the dial on the tool, move the thin wire in
or out of the instrument until the wire vibrates
into a fan pattern. Continue to move the wire in
or out until the fan pattern is as wide as possible.
When fan pattern is as wide as you can make it,
read the engine speed from the scale on the face of
the instrument.
A digital tachometer can also be used to check
engine rpm. See Figure 13-26. The lead from this
particular tachometer is simply wrapped around
the spark plug wire. The tachometer will display
engine rpm when the engine is running.
Another convenient and accurate method of mea-
suring rpm is with a portable optical tachometer like
the one in Figure 13-27. Before using this type of
tachometer, a small piece of refl ective tape is placed
on a rotating part of the engine such as the crank-
shaft, fl ywheel, or pulley. The optical tachometer
is aimed at the rotating part. The rpm is read on
the digital display in the window. This tachometer
can measure from 5–100,000 rpm at a distance up
to three feet and at an angle of 45°. These tachom-
eters are small enough to fi t in a pocket and weigh
only 6 ounces. This is a safe method of measur-
ing engine rpm, because contact with a hot engine
is not necessary and a safe distance from moving
parts can be maintained.
Figure 13-25.
You can measure the speed of an engine through the
vibrations caused by power pulses of the piston.
RPM scale
Vibrating wire
Figure 13-26.
A digital tachometer can be used to check engine rpm.
Lead to spark plug wire
Digital tachometer