Starting at the top:
The cap. I doubt the cap is rotating since you state that the timing is off.
A rotating cap would produce rotor misalignment, but not alter the timing.
The points plate. I assume your distributor is mechanical advance only and
the plate shouldn't rotate.
The points. If the gap is not changing then they are not likely the problem.
The mechanical advance shaft. If the lubricant (or lack of) is causing
binding then it could be a contributor to settings changing. Hot/cold
conditions could be affecting timing readings as the lubricant (I'm thinking
sticky grease here) becomes more pliable.
Advance weights. A similar condition may exist with binding either on the
weight pivot or the advance contact.
Advance spring. A broken spring might cause erratic advance.
Hold down clamp. I have seen hold down clamps seemingly tighten..., but not
hold the distributor. The bolt may be binding, or the clamp may not be
properly contacting the distributor. If loose enough the distributor could
skip a tooth.
Excessive end play in the shaft. Excessive end play could cause the
distributor to ride up on the cam gear and skip a tooth.
Drive gears. You state the drive gears are good so I will agree others that
the gear pin may be missing.
Cam timing chain. I doubt this is the problem as you seem to correct the
problem with distributor changes. If the cam timing was slipping even
correcting the ignition timing would not correct the cam.
All the best. I think pulling the distributor and opening it up is essential
to getting to the issue.
Tom
----- Original Message -----
From: "Jay Laifman" <jay.laifman@gmail.com>
To: <rfraser@bluefrog.com>
Cc: "'Gary Pointer'" <gpointer@telusplanet.net>; <tigers@autox.team.net>
Sent: Sunday, March 25, 2012 10:01 PM
Subject: Re: [Tigers] Timing deviates - advice needed
> I'm going to ditto the sentiment that something is loose or cracked
> because
> each time you touch the distributor, you "cure" the problem. My
> suggestions
> are the cracked cap that someone else already mentioned and the power wire
> to
> the distributor. The wire could be bad and when you move it, it gets
> better
> contact again, until it wiggles loose again after driving. Also, on the
> distributor, I don't know the Mallory in particular. But, how does the
> cap
> "seat" on the distributor itself? Could it be that the tab, or whatever
> it
> uses, is messed up and the cap itself can rotate too much?
>
> Also, a number of people have mentioned the gear at the bottom of the
> distributor. You said it looked fine. How is the gear held on to the
> shaft?
> Could it be that the pin - or whatever it uses - is broken and the gear
> can
> actually slip on the shaft - but look fine from the outside?
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