Ford 302 runs rough at 2500RPM. What's wrong with it?

It appears to be a vacuum leak.

I switched out carbs today, and it didn't work out because my old holley is a raging piece of ****. But while doing so, I noticed that the rubber vacuum line connected to a port on the intake manifold looked like it wasn't connected snugly. Sure enough, it pulls loose easily. The end is oval shaped, and stiff. So I cut it off, and now the line fits tightly. I take it for a run, and it's much better. It still has a hesitation, but at 2-3" of vacuum.

I think it's because these lines are silicone, they act differently than the rubber ones used to. They leak gradually, and intermittently. Previously with rubber lines, it was a sudden and consistent leak, and very obvious. Lesson learned. Getting some more silicone hoses, and replacing every line on the engine.

 

Just a follow up if this thread came up in a search. Turned out the roughness was worn bearings. The bearings were grabbing at 2500. Later, it was 1800, then 1200 as the wear got worse. Slowly developed rod knock, but kept oil pressure fine. Original cause was overheating. I thought motor was fine, but the bearings must have went dry and scuffed or something.