Disconnect the vacuum line to the distributor, plug the carb port, then check your timing at idle and at about 3500 RPM. You may have to add a timing tape. That should tell you a lot.

Re-check your valve lash. If they're hydraulics, set them at zero lash plus a quarter to a half turn.

Have you checked the float levels?

Demons are usually darn good right out of the box, and your problem sounds like something other than carb.