ENGINE TUNEUP PROCEDURES
There is no scheduled interval for performing an engine tuneup. As long as the engine performance is
satisfactory, no tuneup should be needed. Minor adjustments in the valve and injector operating mechanism,
governor, etc. should only be required periodically to compensate for normal wear on parts.
To comply with emissions regulations for on-highway vehicle engines; injector timing, exhaust valve clearance,
engine idle and no-load speeds, throttle delay or fuel modulator settings must be checked and adjusted, if
necessary, at 50,000 mile intervals (refer to Preventive maintenance).
The type of governor used depends upon the engine application. Since each governor has different
characteristics, the tuneup procedure varies accordingly. The following types of governors are used:
1. Limiting speed mechanical.
2. Variable speed mechanical.
3. Constant speed mechanical.
The mechanical governors are identified by a name plate attached to the governor housing. The letters D.W.-
L.S stamped on the name plate denote a double-weight limiting speed governor. A single-weight variable
speed governor name plate is stamped S.W.-V.S.
Normally, when performing a tuneup on an engine in service, it is only necessary to check the various
adjustments for a possible change in the settings. However, if a cylinder head, governor, or injectors have
been replaced or overhauled, then certain preliminary adjustments are required before the engine is started.
The preliminary adjustments consist of the first four items in the tuneup sequence. The procedures are the
same except that the valve clearance is greater for a cold engine.
If a supplementary governing device, such as a load limit device, is used, it must
be disconnected prior to the tuneup. After the governor and injector rack
adjustments are completed, the supplementary governing device must be
reconnected and adjusted.
To tuneup an engine completely, perform all of the adjustments in the applicable tuneup sequence given below
after the engine has reached normal operating temperature. Since the adjustments are normally made while
the engine is stopped, it may be necessary to run the engine between adjustments to maintain normal operating
Use new valve rocker cover gaskets after the tuneup is completed.
Tuneup Sequence for Mechanical Governor
Before starting an engine after an engine speed-control adjustment or after
removal of the engine governor cover, the serviceman must determine that the
injector racks move to the no-fuel position when the governor stop lever is placed
in the stop position. Engine overspeed will result if the injector racks cannot be
positioned at no-fuel with the governor stop lever.