DaveD
I have a couple of questions:

Has this motor been recently overhauled?
(if so I am stumped follow previous advise)
If not:

Is there a lot of sludge on and around the valves and springs?
(you may have slow oil return - clean out oil return passages)

Good luck


Brian Moore
1949 3100 5 window Deluxe
"Today is better than yesterday, but not as good as tomorrow"