jmac it's the 1200 gx 38mm su flat top carbs, thanks for the input guys, i will get some new needle and seats and give it a go. sure that is the problem because if you drive it's fine on consuption and when you stop it idles fine for a second and then starts missing and puffing feul

Bakat you had me wondering about viscosity and if I did have it back to front, so I spoke to the old mechanic again and also an old school british mechanic at work who worked on MG's, Triumphs, Austin Healys, Rovers etc for years before switching to Honda.

Thinner viscosity does make them run richer. Less fluid resistance = higher piston lift.

Thank you for prompting me to question it. Seeing as I am about to rebuild a pair of SU's myself soon, it gave me the direction to ask a couple of old crusty spanner swingers heaps of in-depth questions, as well as finding out for sure about which viscosity oil suits them best. Respect!

if you have thinner oil in dashpot, the piston will lift faster, so the carb(s) 'choke' will get relatively bigger momentarily than when you use thicker oil. what will happen is, when the choke is bigger the air speed across the choke decrease momentarily when you open the throttle, hence they will suck lesser fuel. i tried using ATF for my twin (not a 1200, but my 510), and black smoke appears. tried using monograde engine oil 40, black smoke appears even when the mix screw bottomed. then i tried just for fun to use no oil in the dash, the engine stutters heavily, a sign of too lean mix. for my application, sewing oil works best.

the dashpot oil acts as accelerator pump. they matters most just after you opened the throttle. they resist the piston movement so the choke become smaller so the air speed become faster (again momentarily),giving higher vacuum signal at the jet to suck more fuel.

a great link for SU (datsuns, not british)

http://www.jetlink.net/~okayfine/sutech.html

i changed both needle and seats, it's still has a miss when idling, but funny thing is it's does not seem to overfeul anymore, i can see the exaust is burning a nice grey white color which is perfect on our feul. so next step i think is a electronic dizzy and coil as this can be the only problem now. funny thing is when you stop it idles smooth for a few seconds then starts sounding like a miss or like its running more on one carb the=an the other.

The latest symptom you have described can be caused by linkages being either too 'bendy' or not having return springs and links 'where' they need to be.

What is supposed to happen is when you let go of the accelerator, the main link returns to zero, but then the carbs can each spring back to the idle position. and on their last 2-3mm of travel, the main linkage doesn't actually touch them (on the better setups - on which note the mini twin SU linkages are fantastic in this area) - that way each carb returns to it's own proper idle rpm spot. If the link arms are all locked solid together, one carb can bottom out on its idle rpm screw but the link arms don't let the other one fully return to it's idle screw end stop. So then that carb opens just a touch more, and with shaking of the engine, vibration, and even heat, it will constantly alter how much throttle that carb has, and it can really send the idle quality to hell.

Then you rev it, it clears, and settles back to idle rpm, then the shenanigans repeat with one (or often both) of the carbs and their idle position.

I don't know what your linkages are like, but I've certainly seen it on a few aftermarket setups (even worse again on triple SUs on local holden 6s, compared to the factory triple stromberg setups on the xu-1 toranas. the su vs stromberg thing is of no real significance, it's just that the factory setup was better linkage wise. Since the aftermarket triple SU setups are way way way cheaper, a lot of people used them, and many had lots of trouble getting the idle to be even borderline acceptable.