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 oflubrication also causes scoring. If the rotor in ahand-oiled system becomes scored because ofinsufficient lubrication, the equipment could be at fault,or the lubrication instructions may not have beenfollowed. To prevent problems in either a hand-oiled orpressure-lubricated system, check the piping and thepassages to see that they are open. When scoring is nottoo serious, lap the rotor and body together. Use a thincoat of prussian blue to determine whether the rotorcontacts the distributor body.A broken spring may be the cause of an inoperativetiming mechanism if a coil spring is used to maintain therotor seal. If the spring is broken, replace it to ensure aneffective seal.An improperly timed rotary distributor will preventan engine from cranking. Use the information given inthe instructions for the specific engine to check thetiming.PLUNGER-TYPE DISTRIBUTOR VALVE.—Ina plunger-type distributor valve timing mechanism, thevalve requires little attention. However, it may stickoccasionally and prevent the air starting system fromfunctioning properly. On some engine installations, thepilot air valve of the distributor may not open, while onother installations this valve may not close. The troublemay be caused by dirt and gum deposits, broken returnsprings, or lack of lubrication. Deposits and lack oflubrication will cause the unit valve plungers to bind andstick in the guides, while a broken valve return springwill keep the plunger from following the cam profile.Disassemble and thoroughly clean a distributor valvethat sticks; replace any broken springs.Faulty Air Starting ValvesAir starting valves admit starting air into the enginecylinder and then seal the cylinder while the engine isrunning. These valves may be the pressure-actuated ormechanical-lift type.PRESSURE-ACTUATED VALVES.—In apressure-actuated valve, the most frequent trouble issticking. The valve may stick open for a number ofreasons. A gummy or resinous deposit may cause theupper and lower pistons to stick to the cylinders. (Thisdeposit is formed by the oil and condensate that may becarried into the actuating cylinders and lower cylinders.Oil is necessary in the cylinders to provide lubricationand to act as a seal; however, moisture should beeliminated.) You can prevent this resinous deposit fromforming by draining the system storage tanks and watertraps as specified in the operating instruction. Thedeposit on the lower piston may be greater than that inthe actuating cylinder because of the heat andcombustion gases that add to the formation if the valveremains open. When the upper piston is the source oftrouble, you can usually relieve the sticking, withoutremoving the valve, by using light oil or diesel fuel andworking the valve up and down. When you use thismethod, be sure that the valve surfaces are not burnedor deformed. If this method does not relieve the stickingcondition, you will need to remove, disassemble, andclean the valve.Pressure-actuated starting valves sometimes fall tooperate because of broken or weak valve return springs.Replacement is generally the only solution to thiscondition; however, some valves are constructed with ameans of adjusting spring tension. In such valves,increasing the spring tension may eliminate the trouble.Occasionally the actuating pressure of a valve willnot release, and the valve will stick open or be sluggishin closing. The cause is usually clogged or restricted airpassages. Combustion gases will enter the airpassageways, burning the valve surfaces. These burnedsurfaces usually must be reconditioned before they willmaintain a tight seal. Keeping the air passages open willeliminate extra maintenance work on the valve surfaces.MECHANICAL LIFT VALVES.—The mechanicallift-type air starting valve is subject to leakage which, ingeneral, is caused when the valve sticks open. Any airstarting valve that sticks or leaks creates a condition thatmakes an engine hard to start. If the leakage in the airstarting valve is excessive, the loss in pressure mayprevent the engine from starting.Leakage in this type of valve can be caused by anovertightened packing nut. The packing nut issometimes overtightened to stop minor leaks around thevalve stem when starting pressure is applied, butovertightening may prevent the air valve from seating.As in the pressure-actuated valve, there may not beenough return spring tension to return the valve to thevalve seat after admitting the air charge.Obstructions such as particles of carbon between thevalve and valve seat will hold the valve open, permittingcombustion gases to pass. A valve stem bent by carelesshandling during installation may also prevent a valvefrom closing properly.If a valve hangs open for any of these reasons, hotcombustion gases will leak past the valve and valve seat.3-26
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