I didn't catch what year tk this was.. ?
I'll go back to what happened on my truck a few years ago. It was running good, then it wasn't. Then it got worse. It almost ran like there was water in the fuel. LOL. Turns out, the star-shaped reluctor in the distributor had broken from rust that had developed on the distributor shaft ..and 'expanded' and broke it. It was not spinning freely, but one or two of the 'spikes' had broken off as I reacall. Plus the reluctor itself was powderly and weak. Also, a couple triangle points on the pick-up coil were badly corroded and worn (corroded) back to a nub. I guess maybe I didn't have a vented cap or the vent was plugge?? When I pulled the distributor apart the pieces fell into my hands ...Its amazing the truck ran at all. Also, those two tiny wires in the picture are known to corrode and break (or badly fray).
There's probably a way to test the signal (check Y/T). The way to inspect is to pull the distributor and remove the gear, so you can pull the shaft out to inspect the parts (beneath the lower plate). A guy can buy quality parts new. I would have rebuilt mine, but, by luck, I found a good like-new distributor on ebay for like $25, a guy wanted out of his shop, and, yeah, ran great after that. Not as bad of a job as it sounds. No guarantees that's your issue, but worth a look if nothing else pans out.
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