Sounds like you know what you are doing.
This is just stating the obvious I guess, but, I would also pull the carbs off and remove/clean out the float bowls and check the jetting and float levels are near standard.
Have a look at the reeds when the carbs are off.
Check for rust in the tank and fuel lines.
Check that the power valve motor works.
Check that (with the cables detached) the power valves rotate without too much force, and set them correctly.
Blow down the exhaust pipes to be sure they are not blocked.