Well, the adventures of my sporadically failing MGB continue. (After
three years of reliability that a Honda owner might envy, I'm not too
disheartened.) The good news is that I *know* it's not the fuel
system; the fuel pump fix has stayed and now that I've got a clear fuel
filter in the line up by the carbs, I can see it working even if I can't
hear the clicking.
But since fixing the fuel system, I've had the problem four more times.
One failure seemed to be due to a loose HT lead from the coil to the
distributor; I shoved that in and the car started up again. The next
failure was a complete mystery, as was the following failure.
That one occurred on the way to Andy's house to work on the race car,
so after I finally showed up we thought about it for a few minutes and
determined that the coil was HOT to the touch. I swapped the coil for
a known good, newer coil and the car started and ran smoothly.
Then on the way home the problem returned. This time I popped the
hood, felt the coil first, and pulled my fingers away from another VERY
hot coil. After a few minutes the car started again and got me home,
but not in a very good mood.
So this morning I checked the Haynes manual and here's what I think is
the matter. The manual suggests that there are two possible failure
modes for the capacitor, fail short and fail open. In the fail-open
mode, the car runs roughly and the points arc. In the fail-short mode,
the car won't run because the juice never gets to the points and the
coil field doesn't collapse. (It also seems to me that this might
result in a hot coil as current would keep soaking into it without
ever discharging, but that's pure speculation on my part.)
I pulled the distributor cap and found some interesting things. I think
I'll pick up a new cap and rotor tomorrow, the existing cap had verdigris
on the plug lead contacts and the rotor was scored from the carbon brush.
More to the point, the ground wire from the points base to the outside
of the distributor is missing almost all its insulation, and the LT lead
from the connector on the outside of the distributor was touching it.
Remember that these are the incredibly flimsy copper wires covered with
what Miq Millman described as flimsy fabric that looks like it came from
an old dirty worn-out pair of cotton underwear. I think the LT lead is
shorting to ground intermittently, the points get no current at that time,
and the coil heats up because it gets a constant soak of current and no
discharge.
In any case, I've already got a new set of points and a condensor to
install over the weekend. I'll stick in a piece of wire from my copious
set of spares, and probably replace the LT lead from the distributor
connector (the one that hooks up to the white-with-black-trace wire from
the coil, if you're following this on the schematic) with good wire at
the same time.
Am I missing something? Does this all make sense? Well, even if it
doesn't, it shouldn't cost much to try it out...
--Scott
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