19-02-2021, 01:22 PM
The dates I gave are very approximate. As a student in the 70's, my 1966 Renault had stuff marked "Bougicord" which was inductive, a friend's 1967 Austin Mini had copper leads with those Lucas right angle plug cap suppressor resistors, other folks with 1960's Fords were "enjoying" carbon string leads.
The carbon brush between distrubutor cap and rotor arm is only tens of ohms and doesn't really add any suppression, it's primarily there as a low friction contact. Some rotor arms on the other hand (especially Bosch) do have suppressor resistors built in, often potted in resin. These are prone to go open circuit.
Any time you introduce an extra series air gap into the HT circuit, it robs some HT voltage to make it arc over. The gap between rotor arm and distributor cap isn't too bad, as it's a very small gap (maybe 20 thou ?) and only under atmospheric pressure. Get a gap in a carbon string lead and it can grow longer and longer until there's not enough HT left to fire the plug.
Before making my "home made" suppressors, I ordered four of the Lucas right angle types, of new manufacture and from an established supplier. One measured 6000 ohms, two about 10000 ohms and the last was open circuit. They went straight in the bin !
The carbon brush between distrubutor cap and rotor arm is only tens of ohms and doesn't really add any suppression, it's primarily there as a low friction contact. Some rotor arms on the other hand (especially Bosch) do have suppressor resistors built in, often potted in resin. These are prone to go open circuit.
Any time you introduce an extra series air gap into the HT circuit, it robs some HT voltage to make it arc over. The gap between rotor arm and distributor cap isn't too bad, as it's a very small gap (maybe 20 thou ?) and only under atmospheric pressure. Get a gap in a carbon string lead and it can grow longer and longer until there's not enough HT left to fire the plug.
Before making my "home made" suppressors, I ordered four of the Lucas right angle types, of new manufacture and from an established supplier. One measured 6000 ohms, two about 10000 ohms and the last was open circuit. They went straight in the bin !