Sorry foe the confusion, I know them as SU's, but they are genuine Hitachi's.
All parts are from the same type Hitachi flat tops.

I notice you keep refering to them as SU's.If they are Hitachi(genuine)and you have attempted to use SU's(genuine)for spare parts then this may well be your problem,as they are close but not a direct swap for parts.

The needles do work properly with the motor and cam, as it has idled with them before.
I re-build the SU's as it was leaking to much air passed the worn bushes and the AFR's would go dangerously lean on cruising. I re-build them from the parts of a second set of SU's that I had, selecting the best parts from the assortment a hand. Their is definetaly less play on the shafts after the rebuild, so I'm guessing they must be better. I've also sealed the shafts on the non linkage ends of the carb's with epoxy to limit any air leaks on to the linkage end of the shaft.

Check for vacuum leaks coming from the bush area the throttle plate spindle in the Hitachi [SU] body. If there is not enough air going over the bridge then no fuel [insufficent amount] will be drawn out of the jet.
Also worth checking the throttle plate as well as they flog out on there sides from years of service.
Can you confirm the correct profile needles are installed to match the engine CC's and Camshaft specs

I need some pointers and help regarding the idling issue I'm having on my A14.
It's an A14 sub, balanced with lightened flywheel, ported and flowed GX head, 280 degree cam and the twin SU's from a 1200 GX.

I the problem that I have, is that I can't get the motor to idle reliably below 1500rpm. I get it to idle for a few seconds at 1100rpm, but then the rpm drops and it dies.

I'm battling to set the timing correctly as I can't get the motor to idle constantly at 900rpm to adjust the timing, but the timing is very close to 6 degrees at the moment.

I've synchronized the carbs, checked the AFR's with my gauge, it's got ATF in the pots, pistons move freely in the pots, spark gaps are correct and the motor pulls like a train when you drive it. But don't try to let it idle.

Any help or pointers on what to check next would be appreciated.