Stuck Valve Check - Quick, Easy, Inexpensive

"Anyway, about an hour and 15 minutes in to the flight, I felt a barely discernible roughness, and then a significant cough, and the RPM dropped about 200. I quickly pulled the carb heat out, but then it got even worse. So I pushed the carb heat back in, waited for a minute, and pulled it back out. My engine was still running, but my RPM I am inclined to think that I was experiencing a sticky valve problem. What do you think John? "

Here is one of my favorite checks as it is fast, easy, inexpensive, and works quite well as I tested it for years in the cylinder shop. I would suggest as part of your compression test you remove the rocker covers and with your two thumbs pushing on top of the valve - pop the valve open and shut. You will feel any dragging if the valve is sticking. If the guide is really worn you will feel two thumps as the valve face closes onto the seat first on one side and then another

There are so many possibilities that only a systematic troubleshooting process will reveal the answer. The O-200 normally is rare to stick the exhaust valve since the original guide material was relatively soft aluminum-bronze. As deposits build-up in the guide, the valve would just wear away the guide. Then Superior had a great idea, use the harder ni-resist guide that the bigger engines have to give better wear - well yes, except now the valve sticks. I'd rather have it wear. It's been so long since I've been in the cylinder repair business that I don't know who is using what guide. 

Do not fly if you suspect a stuck valve. If the valve sticks closed and doesn't open in 1/2 propeller revolution the rocker arm supports will blow-off leading to total loss of engine power. Even if that doesn't happen, with each revolution the high opening forces are smashing your camshaft lobe flat. This is why I do not recommend Marvel Mystery Oil and other solvents to fix a sticky valve - yes they might unstick it in time but while it's doing it's work you risk expensive and dangerous engine damage. Also, the root cause of valve sticking is not corrected. A O-200 engine should not stick valves - if it does then something is wrong with the engine (could be the shape of the exhaust port). 

Camshaft lobe damage. Trying to push open a sticky valve smashes the camshaft lobe ($$$).