engine busting up

replace the power valve, check your timing curve, adjust the vaccum pot if it's connected. Light load is PV territory, and it may be a good one in there, but it may be too low for what the engine likes. If it's running a 5.5, try a 6.5. It really only needs to be shut at idle, so I try for the highest number that will stay shut at idle, in gear. Also check your centrifical springs..maybe one let go, of the set screw holding the cent. limit slipped...