I have an Echo PB 755T backpack blower. The unit runs great for about 1.5hrs then stalls out. If I let it cool 10 minutes, prime it, it starts up and runs for about 30minutes before shutting off again. I replaced strainer, fuel vent and spark plug with BPM 8y, but still have same issue. I noticed the spark plug is very white in color. Any ideas what may be the cause.