There would only be smoke if the valve seal was bad, not the valve. And it would only really smoke on startup but thats not the problem. You need to find the problem. If #3 or 5 cylenders are also low then ou might have a blown head gasket between the cylenders. Other things to do is get a mechanics stethoscope and an air chuck that threads into the plug hole. Pump air into the cylener and listen with the scope in the carberator throat(if you hear air then bad valve) and listen in the dipstic tube(f you hear ar then it's bad rings) also listen around the outsideof the head b that cyl to se if the gasket is blown to the outside.
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