Re: Intermittent running

[Tim Buja <buja@domain.elided>]

Graham Nicholson wrote:
> I have one of those very annoying intermittent faults and would be
> grateful to hear the ideas of the list as to the possible cause.

> The problem only occurs when the engine is warm.  On acceleration in
> high gear at low revs the engine momentarily stops and then starts
> again. Usually this occurs only once but on some occasions will do it
> for three/four times
>...
> Initially I suspected ignition but this has all been replaced/checked
> out.  Since the fault occurs only on acceleration at low revs I now
> suspect a fuel fault.  However I find it strange that a fuel fault
> would have such a sudden stop/start symptom. (could both carburettors
> be wrong at the same time?)

I don't think that a fuel problem will result in the symptoms you
describe, simply because of the short amount of time that it occurs.
Typically, once the engine is running and up to temperature, fuel
system problems have long response times, while electrical problems
can have both long and short response times.  For example, if your
car has a clogged fuel filter, you might be able to get it to start
and run at idle for extended periods, but you won't be able to maintain
high speeds for extended periods of time since the restriction in the
filter will prevent the fuel pump from filling the the float bowls
fast enough.  You also need to look at how a _single_ component
failure will cause this problem.  Dual simultaneous failures will
be rare.

I think you need to look at the electrical system again.  How did you
check the ignition system and rule out any faults there?  Does your
tach drop to zero immediately when the engine cuts out, and then
immediately jump back up when the engine starts to fire again, or
does the needle gradually fall with the engine rpm?  (I have first hand
experience with this because the sudden drop and sudden rise in the
tach indication with the engine still turning is a symptom of an
impending heat related failure of the TR8 electronic ignition module,
wisely ;-)  located inside the distributor.)

Assuming you have points instead of electronic ignition, I suspect
that your breaker point baseplate grounding lead may be failing.
Accelerating in high gear at low revs will put a heavy load on the
engine, with a corresponding loss of vacuum.  A sudden change in
vacuum will cause the vacuum advance/retard capsule to move the
breaker point baseplate suddenly.  The movement of the baseplate
will move the small grounding lead and if it is wearing thin, the
movement might affect its ability to pass current.  Watch your
tach the next time this happens to you.  If the needle immediately
drops to zero when the engine cuts out instead of gradually falling
as the engine rpm drops, then you have an electrical problem.  If
the needle gradually falls, then I'm way off base here and you need
to look elsewhere!

Good luck in troubleshooting your Stag.

Tim Buja - Rockford, Illinois, USA - 80 TR8, 73 Stag, 72 TR6