With the stock solid lifter setup, there are two holes in the cam bearing, and two holes in the cam. The only time the valve train gets oil, is when these holes lineup. I have seen the holes in the cam and the bearings be off, and not line up properly. This will restrict the oil flow to the valve train. I just use a die grinder, with a cutoff wheel, to grind a groove in the cam journal (not bearing). Sometimes this will result in too much top end oil. Before I install the head, I tap the oil feed hole in the rear rocker stand, so I can screw in a restrictor if necessary. Head does not have to be removed to change the restrictor, just the rocker shaft.