It might be totally unrelated but just had a similar fault with a strimmer with the same carb as yours. Tried replacing it with a new one but only made a slight improvement . I removed the carb. again and the spacer behind it so that you could see into the barrel easily. the piston when slowly moved up and down with the pull cord (spark plug removed) appeared to have lost a ring. The owner declined spending any more money on it so not sure if that was a problem or not but I suspected that it was not drawing in enough fuel from the carb. because it had a broken ring that had dropped into the sump.