Having just rebuilt a Hitachi I came across something else to check carefully when doing an overhaul on these Carburettors, Somebody had at some point replaced the throttle housing section and I could not figure out why it ran like crap up to 2000 RPM.

That is until I pulled it apart properly, It was a used Junk yard special just a quick clean with carb cleaner as all the shafts etc were good, that was the first mistake should have gutted it totally first time.
Well it seems you can interchange many Lower throttle housing sections to sort out a worn throttle shaft scenario, But just make sure the holes to feed the progression circuit between the base and the main carb body actually line up with each other these did not so into the spares junk pile and I found a Subaru L series wagon base that fitted and did line up.

Hint the phenolic plastic like spacer between the sections is a good gauge as to whether they are matched properly.

Hitachi carburettors have been fitted to various Mazdas, Nissans,and even some Hondas so beware that with many of these carburettors being quite old there is every chance somebody has been there before and caused a problem not found on a stock factory spec carb.