I'm doing a single carb conversion on my '67 Atlas, with a 2:1 manifold and a Premier 932 that have been supplied by RGM.
On closer inspection I see that the carb is fitted with a stepped spray-tube and a 4-ring needle, which would single it out as being meant for a 850 Commando rather than a replacement for the old Monoblocs on my 750 Atlas.
There are opposing opinions as to the advantages of the stepped spray-tube. Some say it improves low-end performance on any machine, others knock it out and replace with a flat-top as a matter of course.
Should I fit the carb and give it a try, or convert to standard 4-stroke with a new spray-tube and 2-ring needle?
2 ring needle?
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The rings indicate which …
The rings indicate which "number" and taper a needle is.
This is a two-ring needle.
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No harm at all in using the…
No harm at all in using the later Commando carbs with the notched spray tubes.
I set up a 650ss with a pair, and it ran absolutely beautifully.
The notched spray tube was conceived to iron out a flat spot when they were putting the Commando 850 MK2 through it's paces following changes to the exhaust (the fitment of mutes to the peashooters) to quieten the bike down in order to comply with US regs.
The flat spot was recorded when doing the roll-on test that required the bike to safely pickup without bogging down in top gear so the rider could perform a safe overtaking manoeuvre.
It was primarily test rider extraordinaire Bob Rowley that found this out, and worked with engineers to implement the fix.
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The actual rings in the needle is NOT crucial, that is purely for adjustment. What is crucial is the taper on the needle, this will be signified by the number of the needle.
With regard to the spray tube I also am listening to see what/how it affects things?
AHA! Now I am informed, When the mention of rings I assumed (wrongly) this was the circlip rings! But the taper is the crucial bit in all this and of course if you get the right 'paper work' you will be able to see the taper with relation to the 'rings'. Thanks Michael.