After I discovered that disconnecting the vacuum advance cured the stumbling
problem, for that is what it was and not a "miss"....I took the car down to
Laurence Anderson, Rolls Royce/PreWar British mechanic extraordinare in
Berekey. It ran like a scalded ape with out that vacuum connected!! Larry
took about 10 minutes to conclude that while he could not explain why it had
run great before and then started this stumbling, that he thought he knew
why. The carbs I have are for a smog set up and the little vacuum tube comes
out the rear one just in front of the throttle plate...he had a big
explaination for this which I didn't understand but said that the right place
to hook it up was on the crossover/balance tube on the inlet manifold. So we
made up an adapter and hooked it up, timed it and away I went, no more
stumbling. To finish up, we hooked it up to his fancy tune up machine, and
took a look at the dwell and the spark on the oscillascope. Dwell was very
steady and the waveform of the spark was excellent which proves that the hall
effect "Ignitor" from SAE was doing a very fine job. ... and what ever shaft
wobble there was, was not reflected in changing dwell angles.
My own theory and Larry didn't necessarily buy into this, is that the problem
resulted from a positive feedback situation where in the power pulsations
through the carbs (ever seen your car under load at high speed on a dyno? an
absolutely frighting display of "gas" exudes/pulsates outward from the
carbies. You wonder how they could be putting a net positive airflow into the
engine....normal sez Charlie Rockwell) got fed back into the advance
mechanism via the vacuum and created the stumbling. It was happening right at
the point of maximum advance. Maybe that is why Cooper S's used mechanical
advance only. I cannot explain why this only started now but I think that
is what it is and the price one pays when one cobbles up non standard
parts...............at least I know that at this point all other components
in my system are ok.....and no more points or condenser to worrry about.
BTW, going back to the points and condenser is very easy incase of Ignitor
failure and would take about 5 minutes, so I just carry the old points plate
with me.
Cheers and happy motoring and again tnx to everyone for their
suggestions.....Hornable mention to TCSyd who insisted it had something to do
with the advance mechanism but was betting on a plugged vac pipe!
Terry (Happy days are here again) Sanders
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