timing is given in degrees Before top dead centre (Btdc) or After top dead centre (Atdc). in order to completely burn the air/fuel
mixture in the car’s cylinders, most timing is such that the spark occurs at a point several degrees Btdc (for example 4
°
Btdc). this
ensures that the full power of the explosions obtained (see fig 1).two additional terms used when describing timing are ‘Advanced’ and
‘retarded’.
As shown in figure 1, when the timing is advanced the spark will occur Btdc. on some late model cars equipped with various emission
control devices, the timing is retarded so that the spark occurs Atdc. timing is changed by adjustment of the ignition distributor.
in order to allow setting and adjustment of the engine timing, timing marks are provided on each engine during assembly. in most cases,
these marks appear on the engine vibration damper or fan pulley at the lower front of the engine (figure 1). on some early engines, this
mark was shown at the rear of the engine on the flywheel.
3.1.
WhEn tO ChECK timing
the instant of the spark plug firing is determined by the closing of the distributor ignition breaker points and will change any time the
points gap or dwell angle is changed. in addition, normal wear on the breaker point rubbing block will change the dwell and affect the
timing. cars equipped with the new “breakerless electronic ignition system” will not normally change timing since there are no breaker
points. for these vehicles, the timing light can still be used to note changes in timing caused by troubles in the ignition system as well as
for resetting timing when components are changed.
3.2.
timing sPECiFiCAtiOns
As noted earlier, timing requirements vary from engine to engine. the engine manufacturer’s specifications should always be referred to
before making any adjustment. these specifications can be found in the cars owner manual, on the under bonnet decal required on all
cars manufactured since 1968 and in various automotive publications.
4. OPERAtiOn - nORmAl EnginEs
4.1.
locate engine timing mark (see figure 1) and use a rag to clean
all grease and dirt from the mark and the pointer. it may help to
use chalk or white paint on the marks to make them more visible.
4.2.
check manufacturer’s specifications for correct timing of the
engine being serviced.
4.3.
start and run the engine until normal operating temperature
is reached .
4.4.
stop the engine.
4.5.
if specifications require, locate the vacuum line going to the ignition distributor vacuum advance, disconnect it and plug the line. A bolt or
pencil may be used to seal the line.
4.6.
connect the timing light as shown in figure 2.
4.7.
start the engine and operate at normal idle speed. Aim the timing light at the timing mark as shown in figure 3
4.8.
trigger the timing light and observe the reading from the timing mark.
▲
Caution: Use care when working around a moving engine. Keep hands, tools and timing light clear of moving fan, belts or
other moving parts.
4.9.
compare reading obtained in step 4.8
with the manufacturer’s specifications.
if timing is not as specified, readjust as described below.
4.10.
ChECK thE iDlE (tl85 only)
4.10.1. set the knob to the ‘0’ position as in figure 4.
4.10.2. follow steps 4.1 - 4.9.
4.11.
ChECKing thE CEntRiFUgAl ADVAnCE AnD
VACUUm ADVAnCE (tl85 only)
4.11.1. follow steps 4.1 - 4.7 under ’operating Procedures’ but increase the engine
speed to 2000 rpm. trigger the timing light and rotate the knob clockwise
slowly and stop when the timing mark moves to tdc or’0°’.
4.11.2. observe the reading from the advance scale as shown in figure 5
4.11.3. compare the reading with the manufacturer’s specification.
4.12.
ADJUsting thE timing tO sPECiFiCAtiOns
4.12.1. loosen distributor locking bolt located at the base of distributor enough
so that the distributor may be rotated back and forth.
DO nOt
over loosen
or remove bolt, but leave it tight enough to prevent distributor turning by itself.
tl84, tl85 | issue:2 (l) 28/09/17
Original Language Version
© Jack sealey limited
Fig.1
Fig.2
Fig.3
