I have been having trouble with the S for quite some time now. A regular miss which I am fairly sure originates from the old distributor. I have in mond replacing it with one of these new units but I refuse to give up until I find the “smoking gun”. Anyone that maybe can advise me on how to test these old units? I have replaced (more than once) everthing related to the unit but stil can not find the cause of the frequent miss, at all speeds and it feels like it is throwing a spark somewhere. New plugs, new plug wires, new distributor cap, points, rotor, condenser, coil, surpressors, checked springs and weights in distributor, checked earth wire inside distributor, replace wire from ignition to coil, what else??
Here are a few sources for misses. Some new distributor caps are made with materials that can have “tracking” at points inside the cap. New old stock distributor caps can have corrosion in the screws which can lead to misses. This can include the central lead screw found behind the carbon contact. Some new rotors can “track”. Sometimes people put the wrong rotor in and the outside point of the rotor where the feed to the individual spark plugs is made will barely be in the correct rotation during the spark time. Sometimes people install sport coils, these provide no, to little, aid for street engine applications, but they can lead to spark going unwanted path such as tracking.
Measure the ballast resistance, typically the coil has an internal resistor on most of our cars. Compare to specification, somewhere near 3 or 4 ohms is common. If possible, check the condenser capacitance. If you can’t measure the capacitance, you can try swapping out 2 or 3 condensers using the reasonable hope they can’t all be bad. While they rarely fail, absence of the condenser acting correctly in the circuit will reduce the spark voltage considerably, typically to just able to fire the plugs but not always.
Check for visible sparking in the engine bay when the missing occurs. This may be done in a dark garage at night.
Some misses have a temperature dependent response, as the distributor parts warm up the missing starts, when tested cold in the garage all could be fine in some cases.
the cost of a digital automotive oscilliscope has fallen to around US $200 (Hantech)
a scope is undoubtedly the best diagnostic tools for ignition systems, as far as I know.
These new digital scopes also come pre-programmed with applications suitable for testing sensor problems on modern vehicles, of course they are Chinese copies of technology that previously cost thousands, but I have had some favorable feedback on the Hantech.
worth consideration for the garage, especially if you fix modern makes & models