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Twin cylinder Briggs engine. Check it for spark on both cylinders cold. If it has spark, then I would want to check the valve setting on the dead cylinder. .004 to .006 on both intake and exhaust. Can be low compression , how many hours on the engine?
Cylinder warms up and will start firing, not that uncommon. For cylinder to fire, it has to have spark,fuel and compression. So you are missing one of the three!