Dist cap leaking: Long

["Mike Nakamura" <mike_nakamura@domain.elided>]

Vehicle: 71 Montreal.

Equipped with Perma-Tune 911 ignition modules, Bosch Blue coils and standard
Marreilli distributor S 127C.  Very close to stock compression (heads shaved
just enough to clean up flat).  Compression check show 180 - 190 per
cylinder.  Leak down results in two sample cylinders # 1 and # 5 are 3-4%.

Diagnosis and repairs work done at Bellevue ALFA Romeo and Ingram
Enterprises, both well respected shops.  We have been very happy with
Bellevue ALFA (worked on our 4 ALFA's over 6-7 years) and are having Wes
Ingram fine tune the SPICA injection on the Montreal.

The car runs fairly nicely without apparent missing, but is running rich.
Loads up plugs a bit, blows black under hard acceleration after low speed
running (too much).  We want to refine the injection to reduce emissions and
maximize performance.  This car should spec easily within the WA state
requirements for 1971.

I have been driving between repair attempts, a total of about 350 miles
since we started looking closely at this problem.

Before we finalize the fuel mixture we want to get the ignition working
correctly.

Following are symptoms and solutions attempted in chronological order >

At idle a large spark jump occurs from the cap to the firewall, 3/4" (19mm).
Easily visible in shop light and accompanied by a loud snapping sound.
Inspect cap, clean cap, check points, clean & gap points, clean rotor.  No
perceptible change.
The cap "looks" good, but there may be small traces or cracks.

Replace cap, rotor and plugs.  Now we have plug wires leaking voltage.

Change plug wires.

Now we have the cap to firewall sparking back.  Mind you, even with this
loss of voltage, we hear no missing and an exhaust gas analyzer does not
show any spikes in HC.

OK, we index tdc to the distributor cap & rotor.  No problems noted.  We
carefully refine the timing and even advance and retard slightly, no change.

We contact several Montreal owners and ask for advice, no one has had this
problem.

We contact Perma-Tune.  These are a well known and accepted replacement for
the original ignition units.  Perma-Tune feels this combination of
components should work well.  No further suggestions.

Upon close inspection, we now notice a small burn mark appearing in the top
of the cap near one of the coil towers.

We detect some voltage leaks around several of the plug towers on the cap.

Now what?

Some one has suggested the dielectric in the caps is relatively weak and
this leaking of voltage is not that rare.  Their solution: Replace points
type ignition with newer technology somehow eliminating the possibility of
high voltage leaks at the cap.  I don't quite understand how to eliminate
the voltage from the cap, but I am not up on new systems.

Could the distributor be worn to the point of allowing variance between the
rotor position and cap contacts internally, thus miss firing the spark out
of sync?

How could the points miss-time the firing of the spark?

Is there simply too much voltage for this cap to hold?

I have a parts manual and we do not see any spacer or other small part
missing.

Thanks for the help.

Mike

Mike_nakamura@domain.elided

Fall City WA USA (~25 miles / ~40 KM East of Seattle)

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