Distributor & rotor maladies!

["John Fielding" <johnf@domain.elided>]

Hi All,

I jsut want to pass on a little piece of information to prevent anyone from doing
something wrong when servicing their distributor and rotor arm.

In a recent post from a digestee he said that he pulled the distributor cap off and the 4
brass posts inside and the rotor tip had black burn marks on them, so he cleaned them off,
presumably with some abrasive?

Well, it is quite normal to find carbonising of the distributor posts and the end of the
rotor arm.  What most people don't realise is that cleaning it off actually is wrong!  The
carbon built up helps to keep the gap between the rotor tip and the posts at near zero
clearance, which helps with the spark energy.  The carbon in fact acts like a radio
interference suppressor and reduces the amount of rfi radiated from the distributor cap.
The carbon is a by product of the brass alloy used when subjected to a corona discharge,
the carbon comes from the CO2 in the atmosphere.

Also if you examine a Bosch rotor arm carefully you will find that a resistor actually
resides within it.  If you look carefully at the centre of the rotor arm you will see a
brass button which the carbon brush in the cap rubs on to transfer the high voltage.
Looking at the tip of the arm you again see a piece of shaped brass, but the portion in
between the centre and the tip has a trough filled with an epoxy.  Inside this trough is a
thick-film resistor of 5k-ohms.  Its purpose is to reduce rfi radiation and as a secondary
role it limits the peak spark current to reduce rotor tip burning.

If the resistor in the rotor arm goes open circuit then the spark voltage available to the
plugs drops to a much lower value causing poor running.  Check this with an ohmeter when
you R & R your distributor!

So the message to all is NOT to clean the carbon off, you are doing more harm than good as
with time the tip of the rotor and the posts will grow an increasing gap and your spark
voltage at the plugs will gradually drop.  Persistent cleaning as part of a service
aggravates the situation as the gap gets enlarged every time you scrape off the carbon and
part of the brass, exposing fresh brass to be burned away again.  The carbon build up in
fact reduces the burning away of the posts and rotor tip to virtually zero.

In pre-war british cars fitted with Lucas distributors the cap had several discrete
resistors mounted in the cap to reduce radio interference.  In those days "radio" meant
long
wave and medium wave, pre FM broadcast days, and these LW & MW recievers were far more
susceptible to ignition interference, hence the extra resistors fitted to Lucas fitted
cars.

These resisitors were a constant cause of misfiring as they had a poor reliability and
often broke down, especially in damp weather - very common in Britain - and soon after
Lucas dropped the idea in favour of plug top suppressors and later resistive ignition
leads, which we still use today.

Just thought you would like to know!

John
Durban
South Africa
Alfetta 1.8L turbo
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