What you need to look out for is when you leave the throttle (do this without the engine running and with the filter off), while looking inside the carb, that the carb closes as much as it can (there should be a 1.5mm-ish gap). If it does not, adjust your throttle linkage so it does. If when you then apply brake, the carb closes a little bit more, then the linkage needs adjusting. This would not necessarily affect forward movement. It's unlikely that the servo is either too weak to center or centers so badly.