The stock Hitachi carb is a problematic nightmare.
There are so many prospective problems that can occur with it that it is usually replaced once it starts having issues.
Do a search and download the vacuum diagram. Then make sure your vac system is routed correctly, And that you don't have any vacuum leaks.
If that doesn't help, my best advice is to replace it with eather a Toyota corolla 1200 carb , Webber , or CV....
There are MANNY good threads on this site to advise you further....Boxcar...