Its totally possible a burnt exhaust valve would do that. Or a cam thats slightly out of time relative to the crank. I would suspect the valve first. You can test it with the engine off, blow some compressed air into each cylinder one at a time thru the spark plug hole. Listen for any hissing in the manifold. See how long the cylinders hold pressure with a leakdown tester, available at summit racing.
***--- Wrecking Crew member ---***
"I've prolly been around longer than you think"