Many of the same issues that cause a poor idle will also affect the carburetor at high speeds.

Most common issues

• 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 nuts 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