Loosen them up to prove the point. You aren't going to frag it on a short mild toot around the block with sloppy valves. The whole idea here is to give them plenty of slack to make sure they aren't being held open by a high spot on the tappets. If it's clattery but idles well, and an intermittent staller when "correct", they there likely is a low spot on the tappet.
BW trickily "resurfaced" some of his in place with a Dremel, but if that doesn't work for you there are always new extraction/install tools, tappets & guides available from H's. Mine are a bit funky but the cams are worn also, so the whole shebang needs to be fed new bits really. Anyhow - loosen them up and see if it makes a real difference, just to definitely eliminate that particular possibility.
The De-Carbon Dance is another good idea - if it's really crudded up, chunks could be flying off and holding open the exhaust for a stroke or two until the valve chews them up. Try the spritzer bottle Steam Clean routine. Peep into the plug hole before & after to see if anything changes.
Do one, then check thoroughly. You really need to know which "fix" worked. Keep after it!
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