DIAGNOSIS The engine is running too lean at high speed. This is a fuel delivery or air intake issue. The most common cause is a clogged fuel filter in the tank or an incorrect carburetor high-speed (H) screw setting.
FIX Check the fuel filter first. Remove the fuel line from the tank and inspect the filter. If it's dark brown or clogged, replace it. Next, check the carburetor adjustment screws. With the engine warmed up, turn the H screw clockwise until it lightly seats, then turn it counter-clockwise 1 full turn. This is the baseline setting. Test the throttle response.
IF THAT DIDN'T WORK The carburetor's fuel pump diaphragm is likely stiff and not pumping enough fuel. The saw uses a Zama C1Q-S267 carburetor. The diaphragm kit part number is 530035185. Replace the entire diaphragm and gasket set. Also check for air leaks at the carburetor mounting gasket.
HEADS UP Adjusting the H screw more than 1.5 turns out from seated can cause a dangerously lean condition and engine seizure. Always retune after replacing any fuel system component.