Glowing headers

When you break in an engne pay ZERO attention to the timing curve, initial or any other nonsense where to set it at idle.

Start the darn thing, turn up the break in rpm, then hit it with 36* of timing.

Doing it the way you may drive around on the street, a slow curve is going to create this issue.

The carb likely isn't on the main jet at that RPM and load.