Sounds like a leak in the injector, or one that is not spraying right. Also there could be a lot of carbon buildup on the piston top. The combination of fuel not being burned completely and soaking the carbon on the piston could keep it running a little bit longer. The carbon stays hot and ignites the fuel even without a spark.
They make cleaners for that purpose at the parts houses, but sometimes the carbon is very hard and takes a wire brush. Just my guess though.
Russ
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