Setting Ignition Timing

 In Engine - Tuning, MGA-MGB, Midget

The following notes are intended to supplement the information on how to set ignition timing (by either method) which is found in most good workshop manuals, with particular reference to how to work with distributors which are worn and not performing to original specifications, as is frequently the case in older cars.

There are 2 basic methods of setting timing – static (engine not running) and dynamic (engine running) (also known as stroboscopic).

Static timing

When the cars were new, static timing was quite satisfactory as the distributor would still have been performing as designed. However, at the age most distributors are now at, they have worn and so can vary from the original spec by significant, even major, amounts. (see my page “Distributor advance rate adjustments: compensating for high mileage wear”) Because of this, it will be difficult to get your ignition set up properly using the static method unless you know more about how your distributor is performing. For best results if you are planning to maintain your car in the long term a strobe will be essential. It costs less than even one bodged “professional” tune-up!

However, static timing can still be a good approximate starting point if your distributor has been removed (for example in an engine rebuild) or you don’t have current access to a timing light (your son borrowed it!), as it should get the timing into the range where it will run reasonably well (unless the distributor advance is seized or totally flogged) and from there you can road test to where it feels best (keep noting what the changes are so they can be reversed if you need to). It is also a very good method of setting your timing if (after a bit of investigation using a strobe light – see later) you know what your static setting should be.

To time using the static method if you have electronic ignition, see further down this page.

Dynamic timing

The simplest form of this is to rotate the whole distributor with the engine running until the fastest idle is achieved. Simple in principle, but quite misleading results are obtained. The problem with this method is that the high manifold vacuum conditions at idle make it respond well to a lot of advance but until the mid ’70’s MG’s had no vacuum advance at idle. Rotating the distributor on such cars to fastest idle makes up for that, but ends up with way too much advance everywhere else. As a result, the engine will pink heavily under load, and may also be checked or kick back when being cranked by the starter due to the spark appearing too soon.

The only useful way to set timing dynamically is with a stroboscopic timing light, (strobe for short!). The principle of this is that the light flashes when the spark appears (thanks to induction from the plug lead) so you can see exactly when the spark is appearing at any particular engine speed.

IMPORTANT: Unless otherwise specified in the manual, strobe figures are always with the vacuum advance disconnected

Strobe timing at idle:

This is the advance figure specified in most manuals. Note that dynamic readings vary with engine speed, so the specified engine speed is most important. The problem with using this method is again that most distributors are high mileage (refer again to my page “Distributor advance rate adjustments: compensating for high mileage wear”). When advance mechanisms wear and are not performing to spec, the result is always correct timing ONLY at the engine speed it was set at – and wrong everywhere else. However, once you have attended to such discrepancies it is a very good method.

Strobe timing at full advance

This is the best method for ensuring best power. Generally, the timing on an engine advances steadily as the engine speeds up – but only to a certain point. After this speed is reached no further advance takes place. The engine is then said to have maximum advance.
To set timing by this method:

  1. Find the engine speed at which max advance occurs.
    Get a helper to gradually increase throttle and engine speed while you watch the advance with the timing light and see where it is when it stops increasing. This can get rather noisy, as some engines don’t get maximum advance till over 4000 RPM (eg Midget) or even higher (eg late MGB’s). Early MGB’s, on the other hand, get maximum advance at 2300 RPM. (Generally, with wear this maximum will occur sooner than original)
  2. To find what the maximum advance should be, add the max centrifugal advance figure to the static timing figure (the figures to use are those in the manual in both cases).    Note: centrifugal advance can sometimes be listed as distributor degrees – if this is the case, double to get crankshaft degrees, as the latter is what you measure with the strobe light.   If you can’t find/work out what the maxium advance should be, 30 deg is a good maximum advance figure to use for standard BMC engines.
  3. Alter your timing to achieve this maximum advance figure.
  4. Road test. If the distributor is badly worn the engine will pink. To eliminate this, you will need to sort the distributor (see that page of mine again) or retard it temporarily until doing so – with a consequent loss of power and economy.
  5. Finally, once you have maximum advance set correctly, allow the engine to idle. Take a note of the engine speed and ignition timing – in future you can use this figure for the strobe-at-idle method safe in the knowledge that your engine will get the right maximum advance figure. It will also save the need for a helper and noisy drama each time you set the timing!
  6. Similarly, make a note of the static setting your distributor is now at for use for initial set-up after those occasions when the distributor has been removed such as to fit new points or an engine rebuild.

Static timing with electronic ignition

Obviously, with the electronic set up you can’t use methods which detect when the points open. The way to go about static timing under these circumstances is:

  1. Mark the current distributor position, marking both distributor to clamp and clamp to block. This will allow you to reference whether the timing was out, and if so by how much. On the Lucas distributors used for XPAG, A,B and C series engines, 1mm at the distributor base is 6 deg on the crank so make sure the marks are clear!
  2. Set the crankshaft to the required static timing point with the rotor pointing towards no 1 plug lead
  3. Refit the distributor cap.
  4. Either attach your strobe light to no 1 plug lead, or (if you don’t have a light) remove no 1 plug lead and fit a spare spark plug to it (with the body of the plug suitably grounded to the engine)
  5. Find the approximate position for the distributor by rotating the whole distributor fairly rapidly clockwise (i.e. opposite to the way to the rotor turns) until a spark appears (be it at the light or at the spare plug). You may have to rotate the distributor anticlockwise 20 or 30 deg first before doing so in order to ensure the timed position is covered on the clockwise sweep.
  6. Having noted the approximate position, refine the process by using a smaller angle of sweep centred on the approximate position and moving the distributor as slowly as it is possible to do yet still get a spark.
  7. Tighten the distributor clamp.

This process may end up with a degree or two of extra advance. In engines with timing marks visible from the top, I would then check by rotating the crank and seeing what the pointers read when the spark appeared. However, this is a luxury for engines which still have the timing marks underneath!   In any event, the right maximum advance usually results in more advance at static on a worn distributor.

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