Technical Information for Gunsmiths and for Classic Car Enthusiasts

MG Restoration

Ignition Timing

Les Bengtson

One of the more basic jobs that the hobbyist can undertake is checking and setting the ignition timing. But, some people seem to feel that this is a complex procedure, best left to the professional tuner. This is not true. It is a relatively easy procedure, requiring simple test equipment that any serious hobbyist should own and be able to use. Hence, I will cover the two basic types of ignition timing, static and dynamic, and the tools necessary to do each.

The two basic methods of ignition timing are static timing and dynamic timing. Static timing is setting the timing with the engine turned off and requires only an inexpensive test lamp. Dynamic timing is setting the ignition timing with the engine running and requires a more expensive piece of equipment called a “timing light”.

All timing starts from the same place. You are going to set the timing to a point where the spark in induced at a specified number of degrees just before the number one cylinder reaches top dead center on the compression stroke. The four stroke engine, the type of engine the MG and most other cars uses, has the crankshaft rotate two times for each time the engine fires. The downwards movement of the piston begins and the intake valve opens, allowing the fuel/air mixture to be drawn into the cylinder as the downward movement of the piston creates a vacuum. This is called the “intake stroke” and fills the cylinder with fuel/air. About the time the piston reaches the bottom of its stroke, the intake valve will close, making a seal and the cylinder is now a closed system full of fuel/air mix. The piston then begins to move upwards compressing this mixture, hence, the term “compression stroke”. When the piston reaches the top of the stroke, the spark plug fires, igniting the fuel/air mixture which burns in a rapid manner forcing the piston downwards. This is where the power comes from and is called the “firing stroke” or “power stroke”. As the piston nears its bottom point, all of the fuel/air should be burned and needs to be forced out, or exhausted, to make room for a fresh charge of fuel/air. Thus, the exhaust valve opens as the piston travels upwards and the rising piston forces the burned residue out of the cylinder on the “exhaust stroke”. Near the top of the exhaust stroke, the exhaust valve closes and the intake valve opens so the whole process can begin again. This is a rather simplified version of what really happens. The valves actually open and close at somewhat different times due to the physics of gas flow and engines have their valve opening and closing times set to provide the best flow of gases, not to fit our idealized model. But, for the purposes of understanding ignition timing, the idealized model works well. One other area which does not fit the ideal picture is when the spark plug needs to fire.

In our ideal model, the spark plug fires as the piston just begins its downward cycle on the firing stroke. But, the fuel/air mixture is a controlled burning, not a violent explosion. The burning begins near the triggering device, the spark plug, and moves across the top of the piston and the combustion chamber until the entire fuel/air mixture is burning. Ideally the full burn should take place when the cylinder has moved slightly downwards, or at about 20 degrees after top dead center (ATDC). You can begin to see one of the problems engine designers must deal with. The rate of burning for the fuel/air mixture takes a set amount of time. This time is a constant and does not change. How fast the pistons are moving and the engine is spinning, does. For this reason, the spark plug is set to fire before the piston reaches top dead center (TDC) on the compression stroke, which allows the mixture to reach full burn at the correct point ATDC. As the engine speed increases, the spark plug must fire sooner so that the mixture will be fully ignited at the correct point to provide maximum power. This is what ignition timing is all about. Setting the firing point of the spark plug so the fuel/air mixture is fully burning at the proper point to provide maximum power. Hence, you can see why, as the engine speed increases, the spark plug must be fired earlier and as the engine speed decreases, it must be fired later. The changing of the exact firing point is controlled by the distributor which has a built in advance system which functions automatically. All we must do is set up the initial point for the system and the distributor handles the rest. Finding this initial point is called “setting the timing”.

Setting the Timing

Static timing is used to set the ignition timing on most of the earlier cars and can be used to set the initial timing on any car. You need to refer to the workshop manual for your specific model, but most cars will start with the static timing set to between seven and ten degrees before top dead center (BTDC). So, the first step in any static timing operation is to set the engine to the required number of degrees before top dead center on the compression stroke. At this time, both the intake and exhaust valves should be closed and the rocker arms loose. (Removing the rocker arm cover and watching the intake valve, the second valve from the front open and close as you rotate the engine over either by using a 1 5/16” wrench or socket on the crankshaft pulley or pushing the car with the engine in gear is not a bad idea the first time you do this to make sure you are actually on the compression stroke. If you push the car, take it out of gear and set the handbrake before proceeding to ensure the engine can not move off the proper timing mark.) The brass tip of the rotor should be pointing towards the number one terminal of the distributor cap at this point. If it is pointing towards the number four cylinder nipple, your timing is “180 out” or you are on the exhaust stroke of number one cylinder, not the compression stroke.

With the timing mark on the crankshaft pulley aligned with the proper timing indicator (pointer) on the timing chain cover, loosen the distributor clamp bolt so the distributor may be rotated. Then, hook up a test light between the coil terminal which goes to the distributor and a good ground. Turn the ignition switch to the “run” position to provide power for the light. Then, rotate the distributor to the left (counter clockwise) about 25 degrees. The light should not be illuminated at this point. Move the distributor to the right (clockwise) until the light comes on. You have now statically timed your engine. It should be capable of starting and running at this point. However, there is an even more accurate way to time your engine—dynamic timing.

Dynamic timing requires two things. First, you must have the dynamic timing specification for your engine. This is given as the number of degrees of advance BTDC at a specified engine speed (RPMs). The second thing you need is a dynamic timing light. I will discuss the different forms of timing lights first, then will discuss how they are used to dynamically time the engine.

There are three types of dynamic timing light. The simplest, least expensive and least effective is simply a light with two long wires, one of which hooks to the spark plug and the other to the spark plug wire. Thus, the firing impulse of the distributor going to the number one spark plug both powers the timing light and the spark plug. This produces a relatively weak burst of light which requires a darkened environment to see clearly. Since many cars have an engine mounted fan on the water pump, and this model timing light must be held fairly close to the rotating crankshaft to be seen effectively, I consider it dangerous to use with most cars. I have used it with my MGA and earlier model MGBs which have the timing marks located underneath the car on the bottom of the timing chain cover. I would not use it on the later MGBs and strongly recommend that it not be used at all.

The second type of timing light has a separate power source and only uses the number one spark plug lead as a trigger. This is a much more powerful light that can be used in direct sunlight, if necessary. There is a ground wire which is hooked to any suitable wire and a red, power wire that is hooked to a source of power. I normally hook mine to the front of the fuse box where the brown wires (constantly hot) go. There is a third wire with this type of light which will attach to the spark plug or spark plug lead. The ones that hook to the spark plug will have a spring with two different sized ends attached. One end will slip over the end terminal of the spark plug and the other will fit into the spark plug lead. Some other forms of this timing light will have an inductive trigger which simply is clamped, loosely, over the spark plug lead. The direct connect method is somewhat less fragile than the inductive trigger system.

The third type of timing light is a modification of the second type. It is powered and connected in the same manner, but has a dial or digital read out on the back. With this type of system, you do not have to use any of the timing marks on the timing chain cover except top dead center. The dial on the timing light is adjusted so the light is flashing when the crankshaft timing mark passes the TDC pointer and the degrees of advance are read off the scale on the timing light. This model of light is more expensive than the basic light without adjustable feature and offers no advantage for initial timing. It does, however, allow you to determine the exact advance at higher engine speeds to check out your distributor function to see how well the actual advance curve matches the factory specified advance curve. Hence, if you do not currently own a timing light, this model may be the best purchase as it will allow advanced troubleshooting and tuning with less work than the basic model.

Using the timing light to set dynamic timing. The first thing to do is to go to your factory workshop manual and look up the tuning specifications. Among them should be a dynamic or stroboscopic timing specification. The one for my 68 GT is 20 degrees BTDC at 1,000 rpm WITH THE VACUUM PIPE DISCONNECTED. Hence, the next thing to do is to disconnect the vacuum advance tube and plug it using a cross point (Phillips) screwdriver or other suitable plug. Lay the plugged tube out of the way where it will not drop down and hit anything or get burned/tangled. (If you have not previously statically timed the car, this is the time to loosen the distributor clamp bolt so the distributor may be rotated to set the timing.) Then, start the car and warm it up so it will idle properly. Adjust the idle speed to the specified speed for timing. (1,000 rpm in this case.) Point the end of the timing light at the crankshaft pulley and see where the engine is currently firing. To advance timing, rotate the distributor body clockwise. You should see the timing mark move towards you. To retard the timing, move the distributor body counter clockwise and you should see the timing mark move upwards and away from you. When the timing is correct, shut off the engine, allow it to cool and retighten the distributor hold down clamp. At this point, you can perform a couple of other checks, not directly related to the initial ignition timing, but useful to know. They are a test of the centrifugal (mechanical advance) and the vacuum advance systems.

To test the centrifugal advance, have an assistant start the car and let it idle. Verify that the timing is still correct (it should be). Then, have the assistant advance the engine speed to about 2,500 rpm as you watch the timing mark. It should move towards you and downwards, indicating that the centrifugal advance mechanism is functioning. If it does not, the centrifugal advance is not functioning and the distributor needs to be stripped, cleaned, lubricated and reassembled. Usually, they are functioning and I have only seen once case, in over thirty years, where they were not. This completes the basic check of centrifugal advance function. This test is followed by the check of the vacuum advance system.

To check the vacuum advance, simply return the engine to idle, remove the plug from the vacuum advance line and connect it to the vacuum advance can. You should see the timing advance beyond your initial setting. (This is why the basic ignition timing is set with the vacuum advance line disconnected and plugged. If you do not do so, the basic timing will be too far retarded and the centrifugal advance curve will continue to be retarded across its range.) There are, however, a couple of exceptions to this test. The 75 MGB used a distributor with a vacuum advance can, but the base plate on the distributor was pinned in place and there is no vacuum advance feature, only mechanical advance. You will, if you have the original distributor, see no advance when the line is connected, assuming someone has hooked a line to the vacuum advance over the years. In the second case is the “Transmission Controlled Spark Advance” or TCSA system found in the last of the MGBs. This system, when hooked up properly, only allows the vacuum advance system to work when the transmission is in fourth gear. Thus, the only thing that will be discovered by checking the system with the transmission in neutral is whether the factory system has been bypassed (giving vacuum advance on all gears).

Some Notes

The use of the dash tachometer to set engine idle speeds is, at best, inaccurate. These instruments are not particularly accurate for tuning use. A better method is the use of a “dwell/tachometer”, a far more accurate test instrument. It is hooked up to a ground and to the terminal of the coil going to the distributor. Most are made to be used with either a six or eight cylinder engine. When used with a four cylinder engine, the eight cylinder scale is read and the figure doubled to give engine speed. Thus, for our 1,000 rpm idle speed, the dwell tach would read 500 rpm on the eight cylinder scale.

While the later, Lucas 45 series, distributors can only be adjusted by turning the distributor body, the earlier model distributors had a “vernier adjustment” fitted to the bottom of the vacuum advance can. At the bottom of the piece holding the vacuum advance can to the side of the distributor is a knurled nut and the letters A R. This feature can be used for minor adjustments of timing without having to loosen the distributor clamp and rotating the distributor. To adjust the timing, simply turn the knurled nut to the left to advance the timing and to the right to retard it while watching the timing mark. When it is exactly on, stop. There is a spring bearing against the knurled nut which will prevent it from turning by itself. Sometimes, this feature does not work properly because it has been neglected for years. If so, make a note of it and clean and lubricate it when you next clean and lubricate your distributor. (Should be on an annual basis.)

The earlier model distributors have a very thin ring of metal where the distributor clamp goes. If you over tighten the distributor clamp, you can crack or break this ring. The Lucas 45 series has a solid ring which will not crack except with massive over tightening. Tighten the clamp just enough to snug the distributor and no more.
Vehicles having the timing marks at the front, underside of the engine (MGA, some MGBs and Midgets) can be timed dynamically if the cord on the timing light is long enough. It simply requires two people, one to watch the timing mark from underneath while shining the timing light and one to adjust the distributor. My 68 GT, with its 18GF engine, was, at sometime in its life, refitted with the later, 18V timing chain cover and crankshaft pulley which has the timing marks where they can be seen from the top, right side of the car. I do not know which versions of the 18G series engine could be similarly modified, but it does make ignition timing much easier and would be worth investigating when rebuilding your engine.

Lastly, always remember the fans and belts. Anytime you are working around a running engine, keep your hair, hands and clothing away from the moving parts to prevent injury.

This monograph may be reproduced only for non-commercial use without other permission of the author. Reproduction for commercial use only by written permission.

Copyright © 2003 by Les Bengtson