I have looked for leaks and not found anything. I used some soapy water to see if it is being drawn in but nothing. I have slowed the idle speed down and readjusted the air bleed screw and yes it is working as I can make the engine run very lumpy with it turned in to it stops then backed it out by 2.5 turns as that seems to be the best place for it.
Yes when over 2.5K the engine will run well up to 4.5K no problem. It will miss fire between idle and 2K but not as bad as 2K to 2.5K.
I till think it's a carb problem as it's always been like it even after removing it and putting it back more that 4 times now to do other things.

Paul.

Paul to clarify; have you removed the pilot jet and made sure the passage is clear? Additionally when it is missing does it do it at half throttle or full throttle etc? I could be totally wrong but it seems like with the mixture screw reset and now working the idle circuit is ok but as you transition from idle circuit to pilot it starts missing and gets really bad on the pilot as that is partially clogged and then as you transition to the main the problem goes away.
Do some testing to isolate the problem; in 2nd gear with the car at 1200 rpms floor it (the motor may stumble if the accelerator pump is not functioning properly) and see if it misses or pulls cleanly. If the miss is gone at wide open throttle repeat the experiment at 3/4, 1/2 and 1/4 throttle. You can tape blocks to the throttle stop to limit the travel to get the desired setting. When the miss returns note the throttle setting. The pilot jet is in play from roughly 1/4 to 1/2 throttle.
If the car misses at all throttle settings at the exact same RPM then I would start checking the distributor; is the vacuum advance diaphragm got a pin hole leak (disconnect it), use a pair of plug wire pliers and pull a wire of each cylinder and see if one is weak. Do you have points or electronic ignition. If its points double check the dwell.
I'm betting on a partially plugged pilot circuit.........again I could be totally wrong.

Tom

Which one is the pilot jet? Is it the one Hitachi calls the idle jet?

Yes I believe what I call pilot jet and idle jet are the same.


Tom

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.

Good point.