Correct Carb Set up. Dommie 99

On a concentric the pilot bush will be a 25 and std for vast majority of 4 strokes so leave as is, for the mains take the monobloc size and reduce by 10%. 106 needle jet, middle notch on the needle and use the slides out of the 928's, should be a 3 or 3.5. As plug chopping is no longer a good guide with modern fuels I tune by going up main jet sizes by steps of 20 until the engine 8 strokes on full throttle and then drop by 10 or 20.

I'm not convinced a pair of 930s will enhance the performance of 99.Too big for the inlet tracts/valves so your gas flow would be sub-optimal. 928's are rather better suited.

Actually, I would recommend a single 376 monobloc or 928. It will give you all the performance you want and don't forget the 99 isn't the strongest motor Norton ever made. I have a small but significant collection of broken con rods and shattered crankcases lurking in the depths of the shed, gathered before I finally fitted an Atlas engine. 

Your 928 carburettors (a little under 1 1/8" bore, which is 28.6mm) are way too bid for a Model 99.  The 650ss is a much more powerful engine, and it runs better on 1 1/16" carburettors than 1 1/8". (It's also a lot more solidly constructed, to handle the power).  The Atlas (750cc) gets 1 1/8" carbs.

If I remember correctly, a 99 gets a single 1 1/16" Monobloc (versus 1" for a 500).  That's a good place to start.

My memory also says that the 99 head is identical to the 88 one.  If so, it has inlet tracts in the head of 15/16" bore.  Whacking ginormous carbs on a fancy manifold is just going to spoil the drivability.

Hopefully yours has the larger inlet valves as fitted 1960 onwards - optional for 1959.  I can't see much point in going for twin carbs otherwise.

A 99 "Special" (Yes - there was one and I have one!) had larger inlet valves, twin 1 1/16" Monoblocs (model 376/67) and HC pistons.  In the original Parts Book for that year the Extras are listed in the back, printed in red type.

Amongst other options for that year were Rev Counter, fully enclosed rear chaincase, front & rear crash bars, air cleaner, folding kickstart (!) and chrome mudguards.  When I corresponded with Norton back in the dark ages, John Hudson recommended the small-bore siamese pipes for the 99 Special.