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720 onan help

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Hey need help. My 720 is only running on one cylinder. It has new plugs and i have good blue spark on cylinder that quits,pulled the head off (bolts were tight but head came right off). I cleaned everything off and put back together, started and ran ok till i revved it up and the same cylinder dropped out. any thoughts? valves are opening and closing

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well went out tonight and changed plugs from one side to the other and the dead side followed the plug. new champion plugs and one bad one. guess i will get a pair of autolites.Thanks for all the responses. Dave

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