My dad has a 1988 toyota pickup truck that backfires when the engine is started and revved at a stand still; it will eventually cause the engine to stall.
Could be valves ... how many miles?
I'm leaning towards a fuel delivery problem. It's flooding out the engine after running a bit. The residue in the cylinders is causing the backfiring when it's started. I don't think it's a dying ECM causing timing problems although that's an option. I'd be curious if there's gas in the oil pan.
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