Cool, Yep when I order from JRC directly, I tell them the application and they jet to suit, and include jets one size up & down as well as a HD spring (Some like the soft spring but most prefer the stiffer one).
Truth is, MOST people seem happy with them out of the box, I am not as active as I used to be but I have yet to have anyone ask for more tuning options. most bolt on and go.
When I set carbs up myself,, I do a variety of steps but I use a old Snap-On tools exhaust sniffer and can verify mixture that way as well as balance between cyls on a twin triple or 4. I like to use vacuum gauges as well but verify with the sniffer exhaust analyzer.
As to the mix screws, correct, would be nice if the casting was machine with blanking plugs so we could use either L or R, or they were handed. BUT JRC makes a small screw that PROJECTS out of the casting, early version had hex flats and you used a small flat wrench, later version was slighty different but still enabled being able to reach and give it a tweak.
I bought a very lightly used set from a local Norton club member as he didnt like having them side by side and not having adjustability. He sold them i disgust & frustration at a fraction of dealer cost so I did buy them and installed on a project. He did not initially purchase from me but an online vendor. I explained and offered the extended mix screws but he went back to concentrics so I concluded a bit of a masochist.
Check with the UK dealer but if not I can supply them to you at cost. Here is a picture of the newer version from JRCs website
See:
http://jrcengineering.com/product-category/fuel-air-delivery/jrc-carb/page/6/
