Is the engine still mounted in an airplane? It is much easier to break engines in on a test stand...

How does the engine feel when you flip the prop? Same as before? If not, any ideas why?

Possibility - fuel line clogged with some kinda gunk, or filter is clogged (you are using one, right?) Or if no filter, some dirt might be blocking the needle valve/spraybar unit. If it is that, pull the needle out and flush the spraybar. When doing that block the spraybar's needle side to make sure you the jets inside the venturii aren't blocked.

Another possibility, but you sound like you may have checked for it, could be a hole in the fuel line. Silicone tubing gets pinholes very easily, and they aren't always easy to spot.

And a rare possibility for new equipment - if the tank is metal and has brass tubing, a tube inside the tank may have split along its length...

Backplate on tight?

Luck! a good Fox 35 is a long-lasting pleasure...