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Ignition timing


									Ignition timing
To make your engine to run really well, the ignition system needs to be able to deliver a
good spark to the combustion chamber at exactly the right time. This means that the
distributor must be set to provide that spark at the optimum moment. Although the
ignition of the fuel air mixture is often described as an instantaneous bang, it is more
helpful to think of it as a burning process. To achieve the best and most complete
combustion or burn the spark plug should fire just before the piston reaches top dead
centre. It is for this reason that ignition timing is described in terms of advance or degrees
before top dead centre (B.T.D.C). At low rpm the piston speed is relatively slow and
there is more time for combustion to occur, so for example an initial setting for an MGB
could be around 13 degrees before top dead at idle. As the revs rise and the piston speed
increases there is less time for the burn to occur, therefore to produce the best cylinder
pressure the spark must initiate the burn much earlier. So that at 3000 rpm for example,
the spark should occur at around 20 degrees earlier, making a total advance of 13 degrees
plus 20 degrees of mechanical advance before top dead centre. These figures are for an
18V engine and it is important to note that settings for timing will vary for different
engines in a model range and also for special stages of tune. Always check your
handbook or workshop manual for the exact figures.

Tools necessary to check the timing of your engine. Home made test lamp, stroboscopic
timing gun, spark plug testers, dwell meter, feeler gauge and screwdriver.

Distributor cap and rotor arm
Carefully examine the distributor cap and rotor arm for hairline cracks, if any damage is
found the cap must be replaced. Although barely visible, hairline cracks will cause
                misfiring and poor starting.

                  Check the inside of the distributor cap for wear and cracks.

Ignition leads
Always ensure that your leads are not cracked or damaged, you can check their operation
and the operation of the distributor by using a set of spark testers fitted over each
sparking plug. These will enable you to see if each cylinder is sparking correctly and to
                 compare the intensity of the spark at each plug.

                  Spark testers reveal the presence and the quality of the spark at each
Make sure that you replace the ignition leads in the correct firing order of 1, 3, 4, 2.
When working out the order of the leads note that rotor arm rotates in anti-clockwise

Vacuum advance
The vacuum advance mechanism adds extra ignition timing at part throttle to improve
economy and performance. You can check if it is operating correctly by disconnecting
the pipe which connects it to the carburettor or inlet manifold and sucking hard on the
end, you will hear a click as the base plate moves. The movement is only very slight and
is best detected by listening for the noise it makes. If sucking hard produces no response
then the diaphragm is probably faulty and replacement will be necessary.

How to set the timing
Check in the handbook or workshop manual for the correct timing figure for your engine,
use the engine number prefix to identify the figures relevant to your MG. The figures
given will refer to stroboscopic/ dynamic timing and also to static timing.

Static timing
This method employs the timing marks positioned at the side of the pulley and on the
crankshaft but the adjustment is made while the engine is stationary. It is necessary to
align the marks while number 1 cylinder is at top dead centre. Number 1 cylinder is the
cylinder closest to the radiator. An effective way help you align the timing marks is to
begin by removing all the sparking plugs, this will enable you to turn the engine over by
pulling on the fan belt or by pushing the car in gear. Using either method you will be able
to turn the engine and position the marks exactly. To be sure that you have number 1
cylinder at top dead centre put your thumb over number 1 spark plug hole as you turn the
engine over and you will be able to feel the air being expelled as the piston rises in the
cylinder. With the plugs removed you can just see the top of the piston through the spark
plug hole as it reaches top dead centre. Now align the appropriate marks on the timing
cover and crankshaft pulley. Check your handbook or manual for the static timing
settings, the MGB 18GG and 18V engine has a static advance setting of 10 degrees
before top dead centre. Look at the row of pointers on the timing cover, the longest
pointer indicates top dead centre and the others are spaced at 5 degree intervals before top
dead centre. Align the marks at 10 degrees before top dead centre with the notch on the
crankshaft pulley . Then connect a 12 volt test lamp (you can make one up from an old
side light unit) between the low tension terminal on the side of the distributor and a good
                   earth on the engine.

                  A test lamp connected to the distributor will light up as the points open.
                  It is made from two crocodile clips and two lengths of wire connected to
a buld holder fitted with a 12 volt bulb
Now switch on the ignition, always remember to switch it off again as soon as the timing
check has been made, because leaving the ignition on for long periods may damage the
coil. When the ignition is switched on, the lamp will light up as the contact breaker points
open and will go out when they close. You have set the marks at ten degrees before top
dead centre and this should be the moment when the points just begin to open, therefore
if the test lamp is already on, the points are open. Should your MG be fitted with a
distributor that has a vernier adjusting nut, you can use this to fine adjust the timing. With
the test light on, turn the nut towards R (retard) until the light goes out, then back towards
A (advance) until it just lights. If your distributor does not have a vernier adjuster you
will need to slacken of the pinch bolt that clamps the distributor in position and make a
very small adjustment by turning the distributor body clockwise or anti-clockwise. Twist
the body of the distributor until the test lamp lights up, you will need to find the exact
position at which the points begin to open and the lamp just begins to light. Once you
                    have located this spot, do up the pinch bolt and clamp the distributor
                    into position.

Test lamp lighting up as the points begin to open.

Stroboscopic timing
This method of dynamic timing involves using a hand held timing light connected up to
the spark plug in number one cylinder. First highlight the timing marks with tiny dab of
white paint or Tippex on the appropriate pointer and on the notch in the crankshaft
pulley. The vacuum advance is then disconnected and its connection in the inlet manifold
covered, then the engine is run at 600 rpm while the pulsing strobe light is pointed at the
timing marks. The flashing strobe effect makes both the timing marks appear to stand still
and provided the engine speed can be accurately measured either by the rev counter or a
device on the strobe itself, then the timing can be correctly adjusted. Adjustment is made
by switching off the engine, slackening the pinch bolt on the clamp that holds the
distributor and rotating it by a very small amount. This is usually a matter of fractional
twisting in either direction until the marks line up. Strobe lights are readily available and
not too expensive, the latest examples have a built-in system of measuring the engine
revs and an easy to fit clip-on attachment for the ignition lead. These new generation
strobe lights require their own power supply which can pose a problem for cars with
batteries located behind the seats. Therefore you will need to connect the red crocodile
clip of the strobe light power lead to one of the output terminals of the fuse box and the
                   black crocodile clip to a good earth in the engine bay.
Hand held strobe timing light connected to no 1 sparking plug.

Dwell meter
Another monitoring device that can improve the performance of the points in the
distributor is a dwell meter. These are easy to obtain and relatively inexpensive, the meter
measures the angle of dwell of the contact breaker points. Using a dwell meter enables
you to adjust the points for the utmost efficiency. Workshop manuals will give the dwell
angle in the tuning data section, the angle for an 18V MGB engine for example is 60
degrees plus or minus 3 degrees.

Time it right
It is vital for the smooth running, long life, economy and performance of your engine to
get the timing correct. If you are not confident that you can carry out the tuning yourself
and you suspect that your engine is not running as well as it should, it is well worth
taking your car to a tuning specialist to ensure that it is set up exactly to the
manufacturer's specification. If your car has been modified then inform the specialist
tuner of all the details because changes to the camshaft, cylinder head or other engine
modifications will alter the timing requirements of your MG.

Ignition Timing
When new cars come off the production line they are clones of one another and the
ignition timing is set to a value determined to be optimum by the manufacturer. I am
going to suggest that for MGB's, the vast majority are no longer clones of one another,
they have changed as has the fuel they burn. Each has experienced different amounts
and patterns of wear, some have been rebored and/or had head skims. Moreover, the
high octane lead laden fuels are no longer available. For these reasons I further suggest
that the text book timing should be regarded only as a starting point and you should
afterward determine the optimum setting for your vehicle and the available premium fuel
(use nothing less than the best).

Before attempting to time the engine, make sure that the advance mechanisms in the
distributor are working as described is the section on Distributors or by using a timing
light. A timing light is not unlike a camera flash gun that is triggered every time a
selected spark plug fires. By looking at a mark on the rotating crankshaft pulley, it is
possible to make the pulley appear stationary, the light only illuminating the pulley when
the latter is in a given position corresponding to the spark on the particular cylinder to
which the timing light is synchronised. As the engine speed is increased, the mark on the
pulley will appear to slide back anti clockwise as the spark advances and the light
flashes a little earlier. Removing and attaching the vacuum advance hose with the
engine revving hard should cause the mark to move noticeably. Be careful using a timing
light, it is very easy to let the cables dangle too close to the fan, alternator and fan belt.

Static Timing
Lean fuel mixtures have a slower burning rate and so require a more advanced ignition
to ensure maximum combustion pressure is reached at about 10° after TDC. There is no
point in trying to find the optimum timing for the car unless the carburetors have been
properly tuned beforehand.
Static timing may be the only timing you need to do and even that can be an
approximate process or totally unnecessary if the car is already running.

Using the diagrams, locate the timing marks on the engine block and the groove on the
crankshaft pulley and highlight them using bright paint. Typist correction ink is now
available in pen form and is ideal.

1. Connect a Circuit Tester (see Tools) between ground and the White/Black wire either
at the coil or at the distributor, whichever is the most convenient.

2. Remove the spark plugs. (For both safety and to relieve engine compression and so
make turning the engine much easier).

The instructions below describe turning the engine if you do not have a 1-5/16" socket
wrench and a breaker bar to turn the engine using the bottom pulley nut. This size
wrench is unusually large for the average tool kit. Incidentally, if you consider investing in
one, it also fits the rear wheel bearing nuts. If you use a socket, there is obviously no
need to put the car in gear and roll it back and forth.
3. Roll the car back

4. Put the car in 4th gear and nudge it forward all the time checking to see when the
timing groove on the pulley is approaching the fixed marks on the block. (4th gear is
selected because it moves the engine more slowly, with more precision and more

5. Switch the ignition ON. (Try to minimize the time you keep the ignition ON, the coil
and contact breaker can overheat). The Circuit Tester light should not be illuminated. If it
is, roll the car back (still in gear) until it extinguishes.

Nudge the car forward until either the Circuit Tester lights or the timing mark aligns with
the 10° mark by the timing chain cover.

Note that this is written based upon a 1967 engine. The timing marks are at about the 5
o'clock position. Other cars may have different looking timing marks in a different
location, especially in later cars where the marks were moved to the more convenient 11
o'clock position. Whatever the number of timing marks, that which is most clockwise is
always at Top Dead Center (TDC). All those behind it are at 5 ° increments and may be
3 or 4 in number. The tine indicating TDC is usually larger than the others, although the
10° mark is often also enlarged.

6. In the event that you go past the mark, back up some so that when you come forward
again, any backlash in the system is taken up.

7. If the light illuminates before the mark corresponds to 10°, then the engine is
advanced and requires retarding.

8. If the light has not illuminated by the time the mark has reached 10°, then the engine
is retarded and requires advancing.
9. Change the engine timing by slackening the clamp bolt around the base of the
distributor and, with the timing groove on the crankshaft pulley in-line with the 10° timing
mark, rotate the distributor clockwise to advance the timing or anti-clockwise to retard it.
At the same time, put gentle clockwise pressure on the rotor arm (to take up slack) and
watch the timing light. When the light status changes stop turning the distributor and
lightly retighten its clamp.

In the case of the 25D distributor, small changes in timing can be made by rotating the
Vernier anti clockwise to advance and clockwise to retard the engine. It takes about 18
clicks of the vernier to adjust the timing just 1°. Counting clicks is tedious, instead you
can mark the vernier wheel with typist's correction ink and work in full turns. There are
35 clicks per turn which equates to 2°. With about 6.75 turns available the vernier can
adjust the timing a little under plus or minus 6.5° from its center point. Whereas the
distributor is turned clockwise to advance ignition timing, rotating the vernier in that
direction retards the timing. This can be confusing but the distributor body, close to the
vernier wheel, carries helpful arrows with the markings A & R for Advance and Retard

10. Try starting the car. If it starts then proceed to 11. If it does not, go back to the
beginning. If the timing checks out OK and the car still will not start, the problem is not
the timing and other causes require checking.

11. With a tank full of Premium fuel drive the car uphill and try to accelerate with out
changing down. Do you hear pinging (pinking) (a tinkling or small rattling sound) that
disappears when you decelerate or change down.

— If so, try retarding the engine until the pinging no longer occurs. If it is evident that
retarding the engine is futile, the pinging persisting but the engine losing power, then you
need to use an octane booster, you local Premium fuel not having sufficient antiknock
qualities. See Fuel. Remember that pinging is an extremely harmful phenomenon and
every effort should be taken to avoid it.

— If there is no pinging gradually advance the timing until it can just be perceived, then
back it off until the effect disappears.

This method of timing, although not text book, results in the best timing for your car with
your fuel, without damaging knock or ping.

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