215"/223" valve lash?

I've been chasing a pop/misfire condition for a while. Investigated carbs and ignition first. I found nothing that would be the cause. The intermittent exhaust pop/misfire continued until it became almost constant. After running a cold compression test cylinders 3 & 4 were 25 psi less than the other 4. I also checked the cold valve lash. On the two low cylinders both intake and exhaust were .012 - .013. The others for were .014 - .015. I've not experienced the lash getting smaller before. Is this phenomena of the lash closing common for the 215/223s? The only cause I can think of is the valves are sinking into the seats. I guess is depended if there are hardened seats installed. Did these sixes have hardened seats?
The lash setting for this engine has me a bit confused right now. As I understand the valve train is made up of the same components from '52 to '59. So why does the lash spec change for different years? I believe the spec for this engine(215) is .015 for all valves hot.
I adjusted all valves to .018" cold. The pop/misfire appears to be cured. I put about 75 miles on it at all road speeds. The top end is a little noisy, but not bad.

Is the .015" lash setting correct for the 215", or should it be set like the later 223" at .015" intake and .019" exhaust.

The 215' s have a little different cam grind (in. & ex .015 hot) is bit more aggressive profile than a 223) you can vary lash a little and experiment to find what works best but the factory settings are about perfect for a stock engine combo. The .018 maybe all right but you should check it once hot to see what it really is warmed up good. Your right that there may be some seat wear to cause it to tighten up but that would not be much different. No hard seats used in any Fords before the 1970's the leaded fuel was used to protect valve guides and seats. Depends how much you drive yours but I check or reset valve lash on my 223 when doing a tuneup. Performance can fall off quite bit with valve lash at a few thousands tight vs. a couple loose. Good luck.

Thanks Bubba,

I set the lash on all valves this morning to a snug .016". The top end is quiet and the performance is back. I'll check them again in 1500 to 2000 miles to see what or if anything changes.