I have a reasonably well used 2GM. Its top speed should be 3600 rpm,
and it has always been able to reach that speed under load. I
normally cruise at 2700, by the way.

This year, the engine will go to 3600 in neutral, but only to 2800
under load.

What I've eliminated -

Its not a control problem, the fuel control on the engine is against
its stop when full on.

I've replaced the fuel lift pump, with no change.

I've replaced all the fuel filters, with no change.

I've run the engine without its air filter, with no change.

Its not a tach problem - in neutral the engine runs at 3600.

There is no smoke at full throttle under load, which makes me think
this is NOT an injector/pre-combustion chamber problem.

I'm not burning oil, although I haven't checked compression. The
engine starts without trouble.

I'm trying to list the possible causes, and ways to do diagnosis.

My list -

A governor problem. If so, I don't know what to look at first, or how
to test.

A high pressure fuel pump problem.

Air leak in the fuel system. (Would that cause this, but no other problems?)

Exhaust back pressure.
--