There should be a small O-ring on the top end of the valve as part of the retainer etc. I have no idea if you can see this when they are together. If they are missing you could have the problem. If the valve guides are really worn and the head is being flodded with a lot of oil, this would also cause the problem. I don't know all the details of your year of engine but some required a restriction on the feed to the rockers to prevent an oversupply of oil to them.
