I was thinking about the possibility that a worn cam lobe might not be making the #5 intake valve open fully.

At cranking speed almost any inlet valve lift is sufficient to full charge the cylinder. Plus of course inlet valves are always larger so the same amount of lift = greater opening WRT an exhaust valve.

With just short manifolds you can easily hear blowing valves on tickover.

Actually this is not necessarily true. If the lobe is worn in a way that changes the duration of it's open position the cylinder will not fill properly and hence will give a low compression. This would show in a leak down test as a cylinder with sound rings and sound valves.

Your advice is good to a point but not necessarily for a do it yourselfer.

And a repalcement cam is a lot less work than pulling a head and a good used cam can be had for probably less than a head gasket set.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

In this scenario, low compression would only be possible if the valve restricts airflow to such a degree that there is a slight vacuum in the cylinder prior to the compression stroke.

I cannot fathom any scenario where this would occur at cranking speed unless the cam lobe is basically non-existent.