Thanks. I have eliminated most of those. It actually has a new fuel tank in it too, from a few years ago. No veg fuel. The line has been checked the whole way. Little carbon overall (had the manifold off not too long ago). Compression is good, etc..

Regardless, none of these could make the exact issue I'm having, in theory anyway. This is "fuel fully flowing, or fuel flowing 80%". There is no middle or gradual increase and again, it's repeatable and consistent. It's really like a valve or switch is opening.