Ttalk Tech

Internal Distributor Failure
August 29, 2010 was an interesting day. Went motoring with lbc folks from the Cape Cod British Car Club. Made a stop after about a hundred miles. Noticed a bit of roughness just before the stop. After about an hour we headed off toward a lunch stop, about 20 miles away. Before we got there Lazarus (my 52TD, always the gent) began to run very rough and with backfiring and refusal to run below 2500 rpm. I decided that it must be caused by the new plugs that I installed the previous day. The old ones were a perfect light tan. Had the old ones in my toolkit. Swapped them back in after lunch. New ones were a bright white! Lazarus wouldn't start. Checked timing - right on; checked for spark - lots of it; checked fuel pump - fine. Someone, i.e., Jack Rosen, spotted that the carbon was missing from the center of the distributor. I just happened to carry a spare carbon and spring, but the old carbon was jammed up in the cap. No sweat, I carry a spare cap. Put it in. Still wouldn't run.
Just before I decided to put the points plate back in place of the Pertronix, Paul Hinchcliffe suggested trying the rotor from his TD. Vrooom! One of the guys with us, Pete Bachand, just happened to spare rotor for his TR3. It fit! Vroom - home we come. The carbon from the failed contact totally screwed up the rotor. Here are some images of the culprits.
The plugs that removed the previous day were a classic, good tuning, light tan.  The plugs that I removed from the engine at the failure were a brilliant white.  After thinking about I suspect that the failure mechanism was causing spark to be applied to all of the plugs at each firing.  Here's an image.  It's not as obvious as I would like it be, but the plug on the left is one that I removed after arriving back home.  The one on the right is one that was removed at the time of the failure:
This is a view looking into the cap where the spring-loaded carbon brush should be protruding from the hole in the center of the cap.  The brush is not only missing, it's actually recessed into the opening.  Notice the carbon covering the inside of the cap:
The engine wasn't about to run until the rotor was replaced.  Here's an image of the top of the rotor.  Notice that it's rather heavily covered with carbon that used to be in the form of a small cylinder in the middle of the distributor cap:
There was a whole lot of sparking going on inside of the distributor.  All standard type of tests, timing, spark at plugs, etc., were as expected.  Normally, I carry a spare rotor (or two), but I seem to recall that I gave my spare to a fellow lbc'er with problems and forgot to replace it.  Live and learn.
NEW INFO AS OF 9/9/10: [Please note that the following images are thumbnails.  Click on them to expand them.]
Put a Lucas distributor cap under the band saw to see what was going on.  Actually, it was just to verify what a bunch of measurements had been telling me.  I suspected that the Pertronix magnetic sleeve was causing the rotor to ride too high in the cap and was totally compressing the carbon brush.  That's an understatement.  The pressure against the center of the cap is so great that it's wearing down the bakelite.
Here's what was going on with the rotor that was part of the catastrophic failure: (It's a thumbnail, click to expand)
Here's the view using a red rotor from Advanced Distributors:
First, without the Pertronix sleeve:(Notice the carbon brush)
Now with the Petronix Sleeve:(No room for the carbon brush)
Here's without the sleeve, or carbon brush to show the gap:
Here's a Lucas rotor (with Pertronix sleeve): (crunch!!)
Here's what you can get by removing .030" from the bottom of a red rotor:(Carbon brush is back where it belongs.)

Here's the cause:
The Pertronix magnetic sleeve is .042" thick at the top to locate it firmly on the rotor of the distributor:
Here's a cutaway of a rotor's position without the sleeve: The rotor is just about touching the cam.
Here's the view with the magnetic sleeve in place: The rotor sits well above the cam.
Obviously, you're free to do whatever you wish, but for now I'm using the shaved down Advanced Distributors rotor.  I will be sending this info along  to Pertronix.
I'd be remiss in not mentioning that the stop we made that day was at Pete Bachand's shop, Kustom & Restoration Specialties in Marlborough, Mass.  Pete is one of that rare breed that can custom tool fittings, components, etc., that are needed in a restoration.  I was very impressed.



(508) 746-6735
Created August 30, 2010
Edited 9/9/2010
and 9/15/2010