Craftsman 20930 Operator's Manual page 9

DIAGNOSING MECHANICAL ENGINE CONDITIONS
LEAKING PISTON RING
Vacuum at idle will be low but steady at about 12 to
16" Hg. Open the throttle and allow the engine to
pick up speed to about 2000 RPM, and then close
the throttle quickly. The pointer should jump 2 to 5"
Hg above its low steady reading. A lesser gain may
indicate faulty rings, and a complete cylinder
leakage or compression test should be done.
BLOWN CYLINDER HEAD GASKET
At idle, the vacuum pump gauge pointer will
fluctuate between normal and a low reading.
The needle will drop sharply about 10" Hg from
a normal reading and return each time the defective
cylinder or cylinders reach firing position.
EXHAUST RESTRICTION TEST
An exhaust restriction will cause normal or near nor-
mal performance at engine idle, but cause very poor
engine performance under load or at higher speeds.
1) Connect the pump hose to an intake manifold
vacuum fitting. Operate the engine at idle and
note the vacuum reading and needle movement.
Compare readings and movements against
descriptions listed for burned valves and late
ignition or valve timing.
2) Watch the vacuum gauge as engine speed is
increased to approximately 2500 RPM.
With motor at idle speed,
vacuum pointer should hold
steady between 16 and 22.
FIGURE 5: VACUUM GAUGE READINGS
With motor at idle speed,
dropping back of vacuum
pointer indicates sticky
valves.
3) An increase in vacuum over that obtained at idle
indicates an exhaust system that is free of restrictions.
4) If the needle drops toward zero as engine RPM
is increased, either an exhaust restriction or an
over-active Exhaust Gas Recirculation (EGR) valve
is causing the problem.
5) Test the EGR valve separately. If it is found to
be in good condition, the problem is a restricted
exhaust. Check and replace if necessary.
INCORRECT IDLE AIR/FUEL MIXTURE
If the gauge needle drifts slowly back and forth at
idle, over a range of 4 to 5" Hg, the fuel mixture is
too rich. A lean mixture will cause an irregular drop
of the needle over about the same range.
INTAKE MANIFOLD OR AIR INDUCTION
LEAKS
If there are any air leaks in the air induction
system, the gauge needle will be about 3 to 9" Hg
below normal, but will remain steady.
LATE IGNITION OR VALVE TIMING
An extremely low but steady reading at idle
indicates late ignition or valve timing, or a
uniformly close setting of the valve lash.
Perform separate tests to determine which
of these problems, if any, have affected
the engine.
With motor at idle speed,
floating motion right and
left of vacuum pointer
indicates carburetor too rich
or too lean.
7
With motor at idle speed, low
reading of vacuum pointer
indicates late ignition timing
or intake manifold air leak.