Distributor Advance Check

The distributor is probably the single most important, and most neglected, component in a conventional ignition system.  Points, plugs, condensers and even leads and coils get cleaned, adjusted and replaced regularly but, as long as there's a spark getting to the cylinders, the distributor itself is generally left to get on with what it does.

Perhaps this is partly because of its name: if it's distributing the sparks properly then there can't be too much wrong, right? Wrong!  As well as the obvious functions of opening and closing the points and sending the resultant spark to the correct cylinder, the distributor on most classic cars provides essential control of the ignition timing.

Since having her, Sheila has suffered from incurable pinking under load.  Pinking is usually caused by having the ignition timing too far advanced or running on fuel with a low octane rating.  Neither setting the timing, nor using 99 octane fuel, made a difference in Sheila's case so it was time to investigate the distributor.

The first thing to do is to make sure that the distributor fitted is actually the right one for the car!  Most distributors are produced with a variety of internal springs and weights to suit different engines.  In Sheila's case, the distributor is a Lucas 25D type unit, which was a very common fitment to a lot of British cars of the era.  All these cars will have their own particular weights and springing to match them to the engine.  The workshop manual specifies that Sheila's distributor should be numbered 41127.  This is stamped on the distributor body:

Distributor model identification

 

It's all very well making sure that your timing is set right either statically or at idle but these setting will be very much too retarded (ie: the spark will be too late) under just about any other condition.  In order to produce the maximum power possible under any conditions, the burning fuel in each cylinder should reach its maximum pressure some time in about the first 1/4 of the piston's down-stroke.

But petrol takes time to burn, so the spark must ignite the mixture some time before that point.  Typically, at idle speed, the spark will be needed when the crankshaft still has a few degrees to turn for the piston to reach top dead centre (TDC).  This is known as ignition advance and, in the case of the Triumph engine, the correct advance is 9 degrees so the basic timing is 9 degrees before TDC (BTDC)

As engine speed increases the spark has to happen earlier to give the burning mixture time to develop full presssure in time.  Similarly, when the throttle is only partly open (most of the time at cruising speed) there is less mixture getting past the throttle which reduces the density of the charge in the cylinder.  This low density charge burns slower than a full charge would so, again, the spark needs to happen earlier if full power is to be realised.

On early cars drivers had a lever (usually on the steering wheel)  which allowed them to alter the ignition advance as they drove but this was soon replaced with a system of automatic advance features in the distributor.

For most cars, the official workshop manuals will specify "advance curves" or particular engine speeds where the timing can be checked to ensure that the automatic advance is working properly.  For the Triumph 1300 engine two sets of data are given:

 

Distributor speed Distributor advance Engine speed Engine advance
1100 rpm +6 to 8 degrees 2200 rpm +12 to 16 degrees
850 rpm +3.5 to 5.5 degrees 1700 rpm +7 to 11 degrees
550 rpm +0.5 to 2.5 degrees 1100 rpm + 1 to 5 degrees
400 rpm + 0 to 1 degree 800 rpm + 0 to 2 degrees

 

Vacuum (in Hg) Distributor advance Engine Advance
15 +7 to 9 degrees +14 to 18 degrees
11 +6.5 to 8.5 degrees +13 to 17 degrees
6.5 +1 to 3 degrees +2 to 6 degrees
4 +0 to 0.5 degrees +0 to 1 degrees

The first is for advance with engine speed (centrifugal advance) and the second is for advance with intake vacuum (vacuum advance). In the manual, the figures are quoted in terms of distributor speed and angles but can be converted to engine speed and degrees simply by multiplying by two (because the distributor turns at half the speed of the engine).

These can be checked on a distributor analyser or, within reasonable limits, by using a rev counter, a vacuum gauge and a timing strobe.  These should be connected according to their manufacturers' instructions.

You also need to arrange a timing scale which goes beyond the one normally fixed to the engine.  In the case of these engines the starter ring gear is conveniently attached to the front of the engine and the teeth are 3 degrees apart.

 

Turn the engine to line up the static timing marks:

 

Static timing marks aligned

 

and provide a suitable pointer aimed at the ring gear.  This gives you a ready made scale in 1.5 degree increments (using the teeth and the gaps as your marks):

Temporary pointer to ring gear

 

Mark the tooth that's beside the pointer with a little paint to provide a reference for 0 degrees of automatic advance:

Mark a reference tooth

 

Disconnect the vacuum pipe which runs from the carburettor to the distributor:

Disconnect the vacuum pipe from the distributor

 

You may need to plug the pipe to the carburettor to get a reliable idle but, on Sheila, this wasn't needed.  You can now start the engine and set it to a slow idle.  The important thing here is to make sure that the engine is running at well below the speed where the centrifugal advance starts to work.  In this case, centrifugal advance starts at around 800 rpm so an idle speed of around 600 is low enough:

Set engine to a low idle speed

 

By shining the timing strobe at your makeshift pointer you should see that it's lined up with the marked tooth.  In Sheila's case the mark had moved away from the pointer by about a tooth (3 degrees), which indicates that the centrifugal advance is starting too early.  This error in itself could explain the pinking but we want to check through the full range so, for now, we move the pointer slightly to line up with the new position of the marked tooth.  The next couple of photos are a bit unclear because digital compacts aren't really designed to take high speed strobe photos!:

Poinre aligned with tooth using strobe light

 

Once the pointer is reset, increase the engine speed to slightly more than the maximum checking point by using the idle speed screw.  In this case, the highest quoted figure is for 2200 engine rpm so I increased to about 2500.  Then drop the speed back gradually to the checking speed and use the strobe to see how far the marked tooth has moved:

Checking the movement of the marked tooth

In this case, the timing has advanced by about 5 teeth, or 15 degrees,  from the low speed point - but this was already 3 degrees advanced of static.  The speed is then dropped to the next lower speed and another reading taken.  Once all the readings are made, repeat and average the two runs.  Including the 3 degree early error, Sheila's results were:

Engine speed Centrifugal advance
2200 rpm + 18 degrees
1700 rpm + 13.5 degrees
1100 rpm + 7.5 degrees
800 rpm + 4.5 degrees

 

These results are all outside the manufacturer's specs but the error appears to be the same (about 3 degrees) at all speeds.  This is probably caused by slack in the centrifugal advance springs or pivots which is allowing the advance weights to start moving before they should.  If the springs were weak or one was broken then we'd expect the error to increase with increasing engine speed.  In any case, three degrees of extra advance could easily be causing her pinking problems!

 

Once the centrifugal figures are taken, it's time to connect your vacuum gauge and repeat the process.  Keep the engine at its slow idle and connect the gauge to the vacuum pipe leading to the distributor using a "T" piece:

Vacuum gauge connected

 

 

Attach a short piece of tubing to the other part of the "T" and suck on it to apply vacuum.  If you find that you can't create a vacuum then check the pipe carefully for leaks.  If the pipe is ok then there is an internal leak in the distributor and the vacuum capsule (or complete distributor) will need to be replaced:

15 inHG vacuum applied by sucking!

 

Holding the vacuum at the maximum level, use the strobe to check how advanced the ignition is then repeat for each of the lower readings.  Sheila's results were:

Vacuum in inHg Vacuum advance
15 13.5 degrees
11 9 degrees
6.5 4.5 degrees
4 0 degrees

 

These readings are too low as the vacuum increases which suggests that the distributor base-plate may be sticking.  This wouldn't cause pinking but would reduce fuel economy at part throttle so needs to be addressed.

 

That completes the checks and has given a good idea of what to look for when servicing the distributor - loose weights or springs in the centrifugal mechanism and a sticky base plate for the vacuum advance.  Servicing the unit, including rectification of these problems, will be covered in a future "How To".

Contact us