Library: MGOC Basic Service 5

Basic Service Guide Part 5

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
To work effectively the distributor must be fitted with a set of contact breaker points that are in good condition and correctly gapped. Remove the distributor cap and examine the surfaces of the contact breaker points. If the points are severely pitted or indented with tiny craters they should be replaced. If they appear to be in good order then check the contact breaker gap. Put the car in gear and push it forward to turn the engine until the cam of the distributor drive fully opens the points, now you can insert a feeler gauge to measure the width of the gap. To adjust the gap it will be necessary to slacken the set screw in the centre of the points so that some movement of the assembly is possible. Then insert the flat blade of a screw driver into the slot at the end of the points just before the condenser and use the dimples in the base plate of the distributor to open or close the contact breaker gap.

Adjusting the contact breaker gap by using a screwdriver to move the contact set against the base plate.

Using a feeler gauge to measure the gap when the points are fully open on the cam, it should be between .014 to 0.15in (.35 to .40mm) for most MGB engines.

Contact breaker points
Removal and replacement is quite straightforward but take careful note of how the points are fitted and the position and route of the wiring. When the new set is in position make sure the distributor cam is holding them in the fully open position and adjust the gap as described above.

Points are fitted onto the base plate of the distributor with a centre screw.

Points are opened as the cam pushes against the sprung contact breaker arm.

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 plug.

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 direction.

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.

Richard Ladds

Castrol Engine Timer
Castrol used to produce a very handy timing wheel of approximately 8" diameter, that fitted over the crankshaft to enable exact static ignition timing.