GMC 4000 Series Service Manual page 269

Sec. 6A
Page 258
GASOLINE ENGINES
Figure 6 - M anifold Heat Control Valve (250 In-Line Engine)
1. To replace the by-pass valve assembly,
remove the housing to cylinder block bolts, then
Subject
Introduction to Engine T u n e -U p .....................
Spark Plug R e m o v a l..........................................
Cylinder Compression Test
Spark Plugs Service and Installation
Ignition System S e r v i c e ...................................
Battery and Battery Cable S e r v i c e ..............
Checking Drive B elts .......................................
Charging Circuit & Unit Inspect. & Adjust.
Manifold Heat C o n tr o l.......................................
Intake Manifold Bolt I n s p e c tio n .....................
Fuel Lines and Filter Service
Cooling System Inspection
INTRODUCTION
Engine tune-up consists of diagnosis, and the
required preventive maintenance performed at
regular intervals to provide maximum perform­
ance and economy.
A systematic procedure must be followed
when tuning an engine. The operations should be
performed in sequence suggested by "Tune-up
Sequence Index" above.
NOTE: When performing work where electrical
term inals could be accidentally grounded, discon­
nect the battery cables so no damage to circuits
will result.
G M C S E R V I C E M A N U A L
TUNE-UP, CHECKS, A N D ADJUSTMENTS
TUNE-UP SEQUENCE INDEX
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remove the housing assembly.
2. Remove the filter element from the housing,
then with suitable drift, drive the valve assembly
out of housing as shown in figure 4.
3. Set new by-pass valve in place at recess in
housing, then with suitable sleeve type installer
(fig. 4) drive valve solidly into recess in housing.
4. Install housing on block using new gasket.
Use plain washers on attaching bolts and tighten
bolts uniformly. Install filter element, and p res­
sure switch (if removed). Start engine and check
filter and housing for leaks. Fill engine crankcase
as necessary to raise oil level to "FULL" mark
on dipstick.
MANIFOLD HEAT CONTROL VALVE
(250 IN-LINE ENGINES)
Referring to figure 6, check for movement of
weight on heat control valve shaft. Shaft should
rotate freely in bushings. If binding is noticed,
lubricate shaft with graphite in alcohol. When en­
gine is cold, the spring should cause valve to move
to closed position. Replace spring if defective.
Subject
Choke and Throttle A d ju stm e n t......................
Instrument Check-out
Ignition Point Dwell
Ignition Timing and A d v a n c e .............. Fig. 17
Charging Circuits - Voltage & Amperage.... 430
Carburetor Adjustments
Idle Speed and Mixture
Crankcase Ventilation Systems
Additional Operations
Cylinder Head Bolt Torque an£
Valve Lash A d ju stm e n ts .....................
ENGINE TUNE-UP
SPARK PLUG REMOVAL
1. Clean all foreign matter away from around
spark plugs and wiring using compressed air. D is­
connect spark plug w ires, and loosen each spark
plug one turn.
2. Reconnect plug w ires, start engine and ac­
celerate to approximately 1,000 rpm. This is done
to blow away loose dirt particles and carbon. Fail­
ure to do this increases the possibility of foreign
material lodging under valves, with resultant false
readings and possible valve damage.
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