Having sorted out the right cylinder cutting out - I wonder if I can further pick the knowledgeable brains on here…
Basically it’s setting the carb up.
I counted the threads of the idle (big screw with spring) and mixture jets when I took the carb apart and so roughly reassembled in the same position.
First thing on start up was high idle so used idle screw to get it down and it seems happy with both cylinders sounding OK.
After a 15 mile ride the right hand (the mended one) plug was obviously running rich. When I compared the mixture jet position to the left hand cylinder, I noticed the left hand cylinder mixture jet was more unscrewed . This seems to indicate you unscrew to weaken the mixture? Is this correct? Seems counter intuitive to me.
I have a colour tune - but am a bit reluctant to use it in an aluminium head as they don’t have that many threads!
I also note a screw on each intake manifold which I am guessing you remove to insert a pipe to balance the carbs - is this via the adjuster under the throttle cables?
Anyway thanks everyone!
Simon
It’s an air control screw, so yes unscrew for more air, less petrol. If it was downstream of the venturi, it would be a fuel regulating screw. At least that’s what the Dellorto manual says. 
You really need a pair of vacuum gauges to do it right. The inlet stubs each have a M6 screw, as you’ve noticed, remove to attach the vacuum gauge hose adaptor (with corresponding M6 thread).
The manuals usually say start with 1.5 turns out for the idle mixture screw, i.e. screw all the way in, then out 1.5 turns. Then start it, get it warmed up, then adjust for best idle. May mean screwing out more. There should be a point where screwing out further shouldn’t make any more difference to idle speed. Need to aim for a position where screwing in causes idle speed to start to drop, then back out the where it was. Do this for both carburettors. (If turning the idle screw does nothing at all, you’ve got a problem, most likely a blockage, then carb needs stripping for inspection and cleaning.) After doing this the idle speed is probably too high, so reduce the throttle stop screws, equally, to achieve desired idle. Engine warming up will increase idle also. Even after thorough setting up, after the first few miles afterward idle may be too high again, so throttle stops need to be reduced some more. Note you’re unlikely to get it to idle throttle closed just after starting, and no choke, only after a few miles of riding. This is normal. Do not try to use the throttle stop screws to get a ‘working’ idle speed without correctly setting the idle mixture screws first. (Guess how I know.) If screws need to be 2.5 turns out or more, could indicate partial blockage of the jets due to accumulated deposits, or fall-out from the petrol over years. Again guess how I know.
With vacuum gauges or mannometer you want to get both carbs near equal Hg with throttles on the stops at idle, and also throttles just open, i.e. slides hanging on the cables. Throttle stop screws and cable adjusters will have to be tweaked accordingly.
Hope that makes sense. On Internet you might be able to find a digital copy of the Dellorto carb tuning manual, which is worth a read.
Thanks! Yup I’m pretty sure what I need to do now.
Bit of interest - they are mixture screws not air. When I screwed them all the way in, the left hand screw end protruded more. Had a good look at the screws and it seems it’s actually the castings have a variation. The little rubber O rings are a bit naff, plus one had lost the thin washer so I am ordering replacements from Eurocarb. In fact - I’m going to strip the left carb as well - overall the bike has at some time been in the hands of someone who really either didn’t care (I guess all older vehicles go through their banger status) or didn’t know!
Done quite a bit of searching and found some good manuals too.
Many thanks!
Yes mixture screws, I was trying to keep it simple.
I’m pretty sure new carbs didn’t have O-rings, relied on screw threads being pretty tight, of course over years wear can take its toll hence recommendation to put O-rings in. I found the O-rings a bit of a pain tho in terms of preloading the coil springs more than I would have liked.