low power when hot?

My theory is that the valve lash needs adjusting.
Mine works really well at .013int/ .023 exhaust, hot lash.

But be careful, the valve stems like to wear ruts in the rockers, so you cannot use the regular-width feelers, which will span the ruts and give excess lash.

You can prove if I'm right by doing a hot-compression test before the lashing. And another after resetting the valves.

If no change, then either I'm wrong, or it's too late, one or more valves is burned. In and case,the test will point you in the right direction.