Many of the same issues that cause a poor idle will also affect the carburetor at high speeds.
Most common issues
- float set wrong
- flange or main body gaskets defective or mating surfaces of parts damaged, allowing an air leak
- carburetor to intake manifold nuts loose
- intake manifold to cylinder head mounting nugs loose
Other possibilites
- primary or secondary throttle valve misaligned in bore
- incorrect choke plate position (should allow choke to open freely when engine is warm)
- retard vacuum pipe loose in flange or leaking
- main jet clogged or the wrong size main jet installed
- malfunctioning power piston
- defective power valve
- throttle shaft worn
- throttle valve misadjusted