Originally Posted by Tib

Not to butt in... I have recently ran into a performance issue with a rebuilt 502. Just a thought but it in my case the valve stem in the heads are now a little long after freshening. When the valves were lapped and valve seats trued up the valve sits a little further into the head marketing the valve stem a little longer on the spring. This will not allow the valve to shut completely which bleeds off cylinder pressure. It may run but not have the full power. The correct way to fix is to shorten the valve stem. Other options are shorter push rods or shims under the roller rocker. Like I said just a thought....

Originally Posted by 28Eliminator

You could also use adjustable rocker arms. Crane, Comps, or more expensive options. That is why there needs to be more testing and indepth troubleshooting done. A vacuum guage would have showed that issue you had also, as would a compression test or a leak down test. Hopefully the OP will get more details and post, as the motor may have a problem even though it was rebuilt.