Hopefully someone will have a good answer. When I sold my Stearman, all my W670 manuals went with it.
About all you can do from the outside is to adjust the valve clearances at the rocker arms. To change the cam timing you have to pull the engine apart (take off the accessory case). If that is the problem, you might also see contact between the valves and the pistons. Look inside the spark plug holes with a flashlight and inspect the tops of the pistons. The pistons may not be able to touch the valves, but if they did, you'll need to remove the valves to make sure they aren't bent and that the valve guides aren't damaged.
Which propeller do you have? The steel bladed McCauley prop has an RPM range that you have to avoid and that should be placarded in the cockpit. I'm not sure, but the other props may have similar limitations. Also make sure that the pin is in place on the prop shaft spline so that the prop is indexed correctly to the crankshaft. There is a blind spline inside the hub that lines up with the location of that pin.
Check the oil screen for metal parts. My engine began to fail when the cage around the crankshaft ball bearings came loose. The balls didn't fall out, but they all moved to one side and allowed the crankshaft to flex and caused a crack. When the ball bearings all moved to one side the engine had a nasty shake that felt like the ignition was failing.
The way I found this was watching outside when someone else ran the engine and I could see the prop hub wobbling.
This failure is what caused me to overhaul my W670 (I did it myself). That was a very long time ago, so check everything I say as my memories may not be very good.
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