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Or a lean condition will cause a backfire......

You could try and read the O2 sensor with a volt meter under cruise. And yes check the ignition too. Make sure the mechanical advance s not sticking while you are at it.

Kevin

Yes, but it will be lean at other speeds also. not just when under load in overdrive. Reading the O2 during the misfire will tell you nothing as when there is an ignition misfire, there is a rush of oxygen which will lead you to believe the engine was running lean.

Over the hill but still climbing.
88 Montero 2.6L auto, manual hubs, cooling fan conversion gps,recurved distributor,LSD Diff.