At the very least, borescope that cylinder, and especially its valves.
Compression may be terrific if a valve never opens!

Rip

John Clonts wrote:
The IO-520 in my Cessna 210 is running rough. It seems to run smooth
at 1000 rpm but the roughness appears above about 1400. One cylinder
is “cold” and apparently not producing any power. I
pulled the rocker cover and found that the intake valve spring was
broken. My mechanic and I replaced it yesterday. But the cylinder is
still not producing. The compression is good (74/80). The spark
plugs are good. He’s going to check the flow through the fuel
injector later today.

Any more ideas? What should we do next?

Thanks!
John Clonts
Temple, Texas
N7NZ