On my IO-540 the manifold pressure has never read over about 28 inches, except when the engine is off. Then it reads ambient pressure.

Most other planes I've seen read closer to ambient pressure at full throttle.

It's been this way since the engine was new. The engine seems to make good power based on fuel flow, temps, and performance, so I don't think it's a weak engine. RPM is fine.

Idle is fine so I don't suspect a manifold leak, but maybe I'm missing something there.

I take MP off the aft right cylinder primer port, which I think is the standard location.

All I can think of is some kind of blockage but there's just not that much to go wrong between the filter and the primer port. I've checked everything I can think of.

Any ideas?