Foreign particles in the air can score the rotor,
resulting in excessive air leakage. You must, therefore,
keep the air supply as clean as possible. Lack of
lubrication also causes scoring. If the rotor in a
hand-oiled system becomes scored because of
insufficient lubrication, the equipment could be at fault,
or the lubrication instructions may not have been
followed. To prevent problems in either a hand-oiled or
pressure-lubricated system, check the piping and the
passages to see that they are open. When scoring is not
too serious, lap the rotor and body together. Use a thin
coat of prussian blue to determine whether the rotor
contacts the distributor body.
A broken spring may be the cause of an inoperative
timing mechanism if a coil spring is used to maintain the
rotor seal. If the spring is broken, replace it to ensure an
An improperly timed rotary distributor will prevent
an engine from cranking. Use the information given in
the instructions for the specific engine to check the
PLUNGER-TYPE DISTRIBUTOR VALVE.In
a plunger-type distributor valve timing mechanism, the
valve requires little attention. However, it may stick
occasionally and prevent the air starting system from
functioning properly. On some engine installations, the
pilot air valve of the distributor may not open, while on
other installations this valve may not close. The trouble
may be caused by dirt and gum deposits, broken return
springs, or lack of lubrication. Deposits and lack of
lubrication will cause the unit valve plungers to bind and
stick in the guides, while a broken valve return spring
will keep the plunger from following the cam profile.
Disassemble and thoroughly clean a distributor valve
that sticks; replace any broken springs.
Faulty Air Starting Valves
Air starting valves admit starting air into the engine
cylinder and then seal the cylinder while the engine is
running. These valves may be the pressure-actuated or
pressure-actuated valve, the most frequent trouble is
sticking. The valve may stick open for a number of
reasons. A gummy or resinous deposit may cause the
upper and lower pistons to stick to the cylinders. (This
deposit is formed by the oil and condensate that may be
carried into the actuating cylinders and lower cylinders.
Oil is necessary in the cylinders to provide lubrication
and to act as a seal; however, moisture should be
eliminated.) You can prevent this resinous deposit from
forming by draining the system storage tanks and water
traps as specified in the operating instruction. The
deposit on the lower piston may be greater than that in
the actuating cylinder because of the heat and
combustion gases that add to the formation if the valve
remains open. When the upper piston is the source of
trouble, you can usually relieve the sticking, without
removing the valve, by using light oil or diesel fuel and
working the valve up and down. When you use this
method, be sure that the valve surfaces are not burned
or deformed. If this method does not relieve the sticking
condition, you will need to remove, disassemble, and
clean the valve.
Pressure-actuated starting valves sometimes fall to
operate because of broken or weak valve return springs.
Replacement is generally the only solution to this
condition; however, some valves are constructed with a
means of adjusting spring tension. In such valves,
increasing the spring tension may eliminate the trouble.
Occasionally the actuating pressure of a valve will
not release, and the valve will stick open or be sluggish
in closing. The cause is usually clogged or restricted air
passages. Combustion gases will enter the air
passageways, burning the valve surfaces. These burned
surfaces usually must be reconditioned before they will
maintain a tight seal. Keeping the air passages open will
eliminate extra maintenance work on the valve surfaces.
MECHANICAL LIFT VALVES.The mechanical
lift-type air starting valve is subject to leakage which, in
general, is caused when the valve sticks open. Any air
starting valve that sticks or leaks creates a condition that
makes an engine hard to start. If the leakage in the air
starting valve is excessive, the loss in pressure may
prevent the engine from starting.
Leakage in this type of valve can be caused by an
overtightened packing nut. The packing nut is
sometimes overtightened to stop minor leaks around the
valve stem when starting pressure is applied, but
overtightening may prevent the air valve from seating.
As in the pressure-actuated valve, there may not be
enough return spring tension to return the valve to the
valve seat after admitting the air charge.
Obstructions such as particles of carbon between the
valve and valve seat will hold the valve open, permitting
combustion gases to pass. A valve stem bent by careless
handling during installation may also prevent a valve
from closing properly.
If a valve hangs open for any of these reasons, hot
combustion gases will leak past the valve and valve seat.