Yes, the 260Z "flat top" SUs are generally despised, but give "common sense" a chance and you may be able to trace the problem and possibly resurrect what most recommend to "toss"
As most are pointing to, it is a fuel issue and most likely that of a mixture problem. Try some of these.
1. Visually check for any possible vacum leaks on the manifold to head side of carbs. Especially cracked hose ends that attach to manifold or route off to other areas nibbles where a rubber hose end could have cracked
2. Observe whether 2 choke cables engage at exact same time. If not, loosen and reset so they pull evenly. (or leave loose if you can get car to idle once warm)
3. Trusting that you have verified that dampener in dashpot are moving freely, including dropping with a quiet "thud" or "clunk" and with the complete air cleaner removed and the car at idle, ever so slightly move the throttle rod center linkage all the while watching both dampers to see if both needles move at the exact same time. If not you need to address that issue so that they move at exact same time.
4. Floats can "sink" over time if they are the black plastic type. If yours have sunk, then fuel supply minimum needed for idle could be restricted. This can be a big can of worms to address so I don't advise opening up the carb to check, unless as a "last resort. It's been so long since I opened up a "Flat Top" that I can not recall which float type they had.
5. Lastly, what often made worn down & aged flat tops run extremely rich in the US and thus caused them to fail a "smog test", was often related to the "power valves" mounted on the side of this SU model variation. It's beyond the time I have to write, but I can tell you that a very experience Nissan factory trained mechanic friend of mine told me that there was a simple trick to often making these overly rich running carbs come back into proper mixture and pass smog.
In a nutshell his technique was to unscrew the side covers of the power valves and rotate the covers to a different position and re bolt them. What he was doing was changing the internal routing passageways inside the power valve. I believe he told me that when rotated to the position he knew worked, one or more passageways were either sealed off or a new one created or both. I'd have to go to my shop and open up one to double check all that, which I don't have time for, but maybe someone else can elaborate and fill in the missing info.
Just some thoughts to look into before you give up on the "Flat Tops".
Eric Neyerlin - owner of ZPARTS.COM, ZCARWORLD.COM and ZCARTRADER.COM
Thanks for posting Eric. not a member of the Boat Anchor Society, which is good to hear!
The power valves, yes, they only fit one way (so the passages run as should- factory) but nothing to stop changing the fuel route. On the ones I have looked at here in the UK, the valves are blocked up. Also, another reason they can run rich, is there is only one central adjustment ( I think mixture) shared by both carbs. Its not difficult to alter so both carbs can be adjusted separately.
Also, like the 240z SU type, they draw additional air through the butterfly spindle (when worn), making tuning even more difficult. This is why Z Therapy put proper bearings in their SU carb rebuilds, to get rid of the worn bit and cut out the excess air.
Correct fuel pressure is a must. There is at least one set of alignment "dots" which are always out on the HMB46W's I have looked at.
All the carbs I have opened have the black plastic floats.
Its been a while since I looked at my paperwork, but like you say Eric, thinking about them with some common sense.
Z club #608
- S53年 ダットサン 260Z (RS30UQ) Ｌ型 改3.2L
- S40年 日産シルビア (日産 1600クーペ) (CSP311)