Atlas Carbs on a 650SS

Leonc · Mar 16, 2025

My 650SS would have originally had monobloc's, but has been wearing concentrics for many years.
These are now at the end of their life as the body wear is excessive.
I have been offered an original matched pair of monobloc's for an Atlas, and reading the specifications they are jetted differently. Also they are slightly larger, but still smaller than my 930 concentrics.
Rather than investing in rejetting, my theory is clean them up and just put them on, see how it goes?
Any experience out their on people having done something similar?

baz · Mar 16, 2025

Sorry I have no direct experience of doing this
But personally I wouldn't hesitate to give them a try
Monoblocs are quite easy to tweak

norton bob · Mar 17, 2025

Recently fitted a 389 monoblock to my 99 , no idea what it was set up for. Runs very well with perhaps a slight loss of midrange urge due to being too large . Not done any plug chops yet !.

robs ss · Mar 17, 2025

Leonc said:
My 650SS would have originally had monobloc's, but has been wearing concentrics for many years.
These are now at the end of their life as the body wear is excessive.
I have been offered an original matched pair of monobloc's for an Atlas, and reading the specifications they are jetted differently. Also they are slightly larger, but still smaller than my 930 concentrics.
Rather than investing in rejetting, my theory is clean them up and just put them on, see how it goes?
Any experience out their on people having done something similar?

The only advice I would have (I have a 650ss) is the if the RH one is chopped then don't use your side-stand while the engine is running - bad for the RH cylinder. My 650ss was not fitted with a side-stand from the factory for that reason.
Float bowl on the LH carb only good for the bike whilst it is level.
Cheers

Leonc · Mar 17, 2025

robs ss said:
The only advice I would have (I have a 650ss) is the if the RH one is chopped then don't use your side-stand while the engine is running - bad for the RH cylinder. My 650ss was not fitted with a side-stand from the factory for that reason.
Float bowl on the LH carb only good for the bike whilst it is level.
Cheers

That is an interesting observation, thanks for the insight.

acotrel · Wednesday at 1:42 PM

With any change of carb, it will always be necessary to adjust the height of the needles. If the carbs worked on another large Norton twin, they should work on a Commando. Main jets are always a bit too rich on any road bike.

Leonc · Thursday at 1:15 PM

acotrel said:
With any change of carb, it will always be necessary to adjust the height of the needles. If the carbs worked on another large Norton twin, they should work on a Commando. Main jets are always a bit too rich on any road bike.

Yes I am expecting that, but will just try them out and see what feedback the machine gives. Cheers

ashman · Friday at 5:57 PM

Jets are cheap and part of tuning, the 650SS are a lot better than the Atlas and running Atlas carbs should be OK, but will have to be tuned in for your motor, you might be lucky just with needle adjustments, but you won't know till you are out on the bike and might have to change jet sizes, not a big deal, all part of tuning, should have smooth running all the way through the revs.

Atlas Carbs on a 650SS

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