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Servicing a vehicle isn't a walk in the spark

By unknown | Mar 14, 2007 | COMMENTS [ 0 ]

Timing is everything.

Timing is everything.

The timing of the spark that ignites the fuel-air mixture in a petrol engine is vitally important.

The spark has to jump before the piston reaches its turnaround point at the end of the compression stroke because it takes a certain time for the flame front, starting at the spark, to spread throughout the combustion chamber.

As the mixture burns, the temperature and pressure in the combustion chamber rise. For optimal engine performance, pressure should peak when the crankshaft has rotated about 18 degrees past the turnaround point (called Top Dead Centre, or TDC for short). To achieve that, the spark has to jump at about 22 degrees before TDC if the engine is running at cruising revs.

Though it might take 40 degrees of crankshaft rotation, the time interval between the moment when the spark jumps and the moment when peak pressure is reached is very brief. Nevertheless, it is essential that we allow for that, otherwise the engine will not run smoothly.

Spark timing would have been a trivial matter had it not been for certain complications.

The first of these is the obvious fact that during starting the crankshaft is rotating much slower than when the engine is running at cruising revs.

If the spark would jump at 22degrees before TDC while the engine is being cranked by the starter motor, the pressure peak would have come and gone before the piston even reaches TDC, and the engine would never start. So for starting and slow running, the timing has to be retarded.

In days gone by, cars had an advance-retard lever on the steering wheel and the driver would retard the timing manually for starting and then advance it with the lever as the engine speed picked up.

But in the distributor design that became the norm for close on 50 years until the arrival of electronic ignition, there was a system of bobweights and springs which automatically varied the timing as the engine speed changed.

During a service the mechanic could adjust the static timing (engine not running) to something like 8 degrees before TDC to allow for easy starting and then, once the engine was running, the bobweights would swing out on their pivots to advance the timing without the driver even having to think about it. Should the engine speed fall, the springs would pull the bobweights back and the timing would be retarded again.

The second complication arises from the fact that the speed of the flame front spreading out from the spark depends on how well the cylinder was filled with fuel-air mixture during the intake stroke.

If the throttle is only slightly open, the piston cannot suck its full quota of fuel-air mixture into the cylinder. Then the combustion takes place more slowly. Thus the timing needs to be advanced for a poorly filled cylinder.

For this purpose an additional advance mechanism, called the vacuum advance, was invented. It has a diaphragm connected to the inlet manifold and also linked to the points plate in the distributor.

High vacuum in the manifold causes the diaphragm to bulge and the points plate rotates to advance the timing.

The basic distributor design had such simplicity and reliability that it enjoyed a long innings.

Servicing it became part of the know-how shared among millions of shade-tree mechanics all over the world.

They would begin by removing the points from the distributor and either clean up the pitting on their contact faces or fit a new set. Then only two adjustments remained.

The gap between the contact faces when separated at their widest by a cam lobe for which you officially need a feeler gauge and secondly, you had to rotate the distributor so that, with the engine static, the points break contact at exactly the right moment, usually with the No 1 piston at about 8degrees before TDC.

The details are described in hundreds of workshop manuals. It's not so easy to be sure when exactly the points break contact, but it was discovered that if you switch on the ignition and place a portable radio nearby, set to shortwave, but off-station so that it emits a hissing sound, there will be a clear crackle at the moment when the points separate. Thereafter, I have found it best to fine-tune the timing by warming up the engine, adjusting the idling to a slow speed, then turning the distributor slightly clockwise and counter-clockwise until the engine sounds smooth, calm and happy.


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