Most troubles are a result of improper adjustment. Generally, this involves the lift of the starting air cam or the timing of the air starting valve. The starting air cam must lift the air starting valve enough to give a proper clearance between the cam and the cam valve follower when the engine is running. If there is not enough clearance between these two parts, hot gases will flow between the valve and the valve seat, overheating them. Since the starting air cam regulates the opening of the air starting valve, check those with adjustable cam lobes frequently to ensure that the adjusting screws are tight. Obtain the proper values for lift, tappet clearance, and time of valve opening for a direct mechanical lift timing mechanism from the manufacturers technical manual for the particular engine. Make adjustments only as specified.

ROTARY DISTRIBUTOR.The rotary distributor timing mechanism requires a minimum of maintenance, but there may be times when the unit becomes inoperative and you will need to disassemble and inspect it. Generally, the difficulty is caused by a scored rotor, a broken spring, or improper timing. 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 effective seal.

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 timing.

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 mechanical-lift type.