Look to your distributer shaft and see if there is a lot of play side to side. If one was looking at the pattern on a scope there would be a dwell variation in a curve.

I have had loose distributers where the load of the ignition points when suitably openned drove the shaft so far to one side that the trigger points didn't open on the other side of the distributer. In such cases the optical trigger is the solution as without the one directional loading of the ignition points the loose shaft will trigger the bottom symetrically loaded points will work.

Just a thought.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician