Is there oil leaking from the bottom of any of the rocker arms (especially toward the front)?
This can be caused by badly worn rocker shafts.
If it were mine, I would turn the idle screw in to raise the RPM to about 800 and see if oil shows up on the front rocker holes.
If that doesn't work, I would take the rocker assemblies off and look at the shafts and the inside of the rocker arms for excess wear, and thoroughly clean the inside of the rocker arms.
When I do this, I remove the end plugs to make sure that they are completely clean. The end plugs are just like freeze plugs and come out and go back in without much trouble. Just make sure that they are snug when you put them back in.
