My 200 has picked up a little stumble. If I hold the throttle a little above idle, for example, it'll run a little unsteady and cough once in a while. Sometimes at idle it'll cough and die. I'm going to check the valve clearances first, then if those are okay, which they very likely are, I am thinking some kind of carb glitch. Any ideas? The air filter is clean and it has a new spark plug. It's got 10,310 miles on it. It starts beautifully.