Echo 600p runaway I have a older echo 600p

[mountainlake]

The boot on mine was cracked. Steve
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[9050LX]

If it's runaway and can't be killed by the ignition switch then it must auto-ignite (a bit like a diesel engine). Could be that the saw runs really hot and some carbon build-up or something is "glowing" and igniting the fuel mix without the spark from the sparkplug. Almost certainly means it is pre-igniting, making it run even hotter to the point that no spark from the sparkplug is needed anymore. Very bad situation for your piston and cylinder imho. The beginning of it running too hot may well be some air leak causing it to run lean (like Paul suggested).
Find the cause and fix it. This is going to kill your saw before too long.

Had an old Suzuki enduro that did this once, opened the throttle and it died.exciting.