Adding a washer won't hurt a thing.

If the spacer and gasket have had to be replaced, you need to make sure they're both installed correctly. You need to make sure the impulse hole is kept open so that the carburetor's internal pump is getting pressure impulses from the crankcase. Otherwise, the engine cannot run correctly because it won't be getting enough fuel.

Also, in cases where the engine's not running well at higher throttle settings, a cleaning of the carburetor's internal fuel screen can help a lot.