Can anyone explain why, when the Atlas is fitted with a single carburettor, a 11/8” (28.5mm) monobloc is used which matches the diameter of the inlet ports in the head (also 11/8”) but when twin carburettors are fitted 30mm concentrics are used? 28 mm twin concentric would seem to make more sense to me as the port / carb diameters would match but RGM, the Haynes manual and the Roy Bacon restoration book all list 30mm. Should I have tapered manifolds to match the 30mm carbs to the 28mm ports?
Thanks for any help, Paul.
Paul, Sorry, this does not…
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Mark, the Atlas had chopped…
Mark, the Atlas had chopped 389 monoblocs until the handed l/h 389 and r/h 689 pairing was fitted in 1966. The concentric only started being produced in 1967.
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Useful Carb Data
The attachment is a handy piece of Carburettor information that relates to the Dominator Twin Models. It comes from the 2020 Dominator Service Notes. Chapter 40. Probably not 'proof-read'.
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spec-ing an Atlas carb
Amal Monobloc data to be found here:
http://archives.jampot.dk/technical/Carburation/Amal_Monobloc__numbers_type_and_use.pdf
According to these data sheets, the Atlas initially used (1963-'4) a 389/87 on the left and 389/88 (chopped) on the right carb, but for '64-'66 used a 389, 689 pairing, with no. 219 or 241 stamping on each (the difference is only the main jet fitted in the factory). For '66-on, they went to Concentric.
BTW I found the data sheets several years ago using a regular search engine. The AJS / Matchless Club have now concealed this page so that it is officially accessible to members only via their website. But if you know it exists, you can use your favourite search engine to find it - or use my link.
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Paul,
Sorry, this does not really answer your concentric question, but twin monobloc 389s or 689s fitted to an Atlas are 1 1/8”, so matching the manifold bore. Perhaps a 28mm concentric is not as efficient as a 1 1/8” monobloc? Just speculating.
Andy