[Brian R.]

Did you check the valves when they are dead cold? Checking them when they are warm is the obvious answer to why they are tight.

If you got those clearances when they were dead cold. I can't explain why they would be so tight except through normal changes possible with that many miles. Since they are all the same, more or less, it seems that it is not a specific problem. It is a general issue that is rearing its head.

Since you can't expect to have your valves stay in adjustment that long. I would forget about worrying why and just get your valve clearances set to the factory specs. What probably happened was over the years, the exhaust valves beat on the seats and caused clearances to tighten up with the cam. I guess this would happen faster if you run lean more often than not. A lean mixture makes everything in the exhaust hotter and softer. What's more, the tighter the clearances on the exhaust valves, the longer they stay open and the hotter everything gets, the tighter they get, etc. - sort of a positive feedback loop.

Just for giggles, I would get a compression check also to test your valves after they are adjusted.