Those may be stock replacement rockers, but you also said they were roller tipped, because of this fact, they may be stock replacement 1.6 ratio rockers, but they also could have also altered the geometry, making the pushrods either too long or short. I would go back and check to see if the geometry has been altered. One other suggestion, is to pull the spark plugs to verify that none have had their gaps closed. I had a 400 awhile back that I had pulled from a F100 and replaced by a built up 400. After about a year, I swapped those engines back, wanting to sell the truck. To make a long story short, the original 400 had a dead miss which puzzled me. I finally isolated the miss, and pulled the plug on that cylinder, the gap had been "bumped" closed, from handling the heads around the shop. Opened the gap, and the miss disappeared. If all else fails, you could posibly have a vacuum leak in the bottom of one of the manifold ports.Cdaniel said: