Running on Three Cylinders

seb fontana

As the title states my 47 CC is running on three cylinders..This happened while going down the road at 40 mph, no hint at anything being wrong prior..No pinging, no over heating, no extra noise..Cyl #4 Just stopped running..Plenty of spark; the plug looked fine but replaced it and still no fire.Valve clearance is at spec of .004/.006.Checked compression and found #30 lbs, all others are at #120..Put air adapter in spark plug hole and hooked air supply up and air was exiting the exhaust pipe..So bad exh valve or seat..Am I missing some thing? There was no clue to something being wrong; I didn't think a valve could go bad in an instant..Any input? 

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