hi glen, what makes you suspect a broken valve spring ? what kind of compression do you have ? do you have a noise?
if you have low compression on a given cylinder, squirt some oil in that cyl. [ plugs removed, if the compression comes up this would indicate rings, if not possible a burnt valve or a valve partially open, as in lifter ? maby
The valves probably need to be re-seated/re-lapped. If you're going to all that trouble, you might as well have the bowls blended and a multi-angle valve job done. It's worth a lot in performance.