Atlas Cafe Racer Dual Carb Selection

mschmitz57

mschmitz57

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I've got a `63 Atlas Cafe Racer.
I bought it with the single Amal Concentric and it's been a nice running setup but I'm beginning to think
it's a little too sedate and it might be time to install dual carbs on the old girl. I've already got 2 sets of Concentrics.
A 930 and a 932 set. The 930's are older types without the drain plug in the bottom but they are in nice shape but probably need
to be cleaned + sleeved. The 932's have been overhauled and sleeved by Lund. I was going to use them on my 850
but I ended-up the a set of PWKs instead. I bought a set of the straight Atlas intake manifolds for just such an occasion.
Will 932's work on an Atlas?

Atlas Cafe Racer Dual Carb Selection
 
I know a 650ss isn't an Atlas, but on my 650ss at various times, I used the original (1 1/8", 1 3/16"??) paired monoblocs, Commando 30mm Concentrics, Commando 32mm Concentrics. The Concentrics were also ran as twin setups and singles, with manifolds matched to the carb(s) and head. The conclusions that I came to were that the single carb set ups, either 30 or 32mm were obviously the easiest to set up/tune, throttle response was marginally better than twin carbs (maybe my imagination). As far as general rideability I preferred the twin 30mm set up, the 32mm was a bit OTT and seemed to produce more noise. I did have to fiddle about with the jets/needles/slides a bit to get the best out of them. I ended up with a huge quantity of jets, needles, slides.
So for your Atlas I would suggest that pretty much anything goes, but any self respecting cafe racer should surely have twin carbs?
cheers
wakeup
 
Yeah, I'll go with the 30mm Concentrics. They would look great with a couple of velocity stacks.
Is there any difference in float height between Commando and Atlas mounted Concentrics?
Or do they sit at the same relative angle when mounted on the manifolds?
 
You could use the 32mm as like the Commando-it depends if you still have the dished 7:5 -1 pistons in there. You really need to taper turn internally 29/30mm to 32mm the spaces. Whatever you decide to do, DO NOT bore out the inlet ports to 32mm in the cyl.head. Also, you might find you have split reducing sleeves in the inlet ports for the single carb. These can be removed.
If you do go to twin carbs YOU WILL lose about 7 mph in top gear at the lower end, i.e. you may not be able to cruise at 15-17 mph in top gear any more. Don’t ask me how I know.
Personally, it’s not worth the effort unless you want to go racing.
 
I just checked my eBay Atlas intake manifolds and the diameter is a bit smaller than the 930 carb bore.
This doesn't look correct to my eyes. Shouldn't the Atlas manifolds match the 30mm bore of the carbs?
 
The inlet ports, (without the split collars ) are probably the same as the 750 Commando 29mm or 30mm depending on the casting. It’s a long time ago when I measured up these heads.
Turning the spacers out to the 30mm size should be no problem.
 
Bernhard said:
You could use the 32mm as like the Commando-it depends if you still have the dished 7:5 -1 pistons in there. You really need to taper turn internally 29/30mm to 32mm the spaces. Whatever you decide to do, DO NOT bore out the inlet ports to 32mm in the cyl.head. Also, you might find you have split reducing sleeves in the inlet ports for the single carb. These can be removed.
If you do go to twin carbs YOU WILL lose about 7 mph in top gear at the lower end, i.e. you may not be able to cruise at 15-17 mph in top gear any more. Don’t ask me how I know.
Personally, it’s not worth the effort unless you want to go racing.
Click to expand...

No, I'm running flat top Hepolite Commando pistons.
 
mschmitz57 said:
I just checked my eBay Atlas intake manifolds and the diameter is a bit smaller than the 930 carb bore.
This doesn't look correct to my eyes. Shouldn't the Atlas manifolds match the 30mm bore of the carbs?
Click to expand...

Curious to know if you have 1" monobloc or 1.5"(long) concentric manifolds. In either case the head end IMO should match the 28.5mm port. In actuality they are probably bigger from the factory, yet at the carb end are small as you say. IMO not worth worrying about.
 
Cruise at 15-17 MPH in top gear? Who tries these things? :mrgreen:

I recall all the old British mags always did a "nonsnatch" test or some such thing. A bike was a worthwhile purchase if it could smoothly pull away from almost walking speed in top gear. I don't know who actually rides like that, nor should one, however the testors always though it terribly important!
 
worntorn said:
Cruise at 15-17 MPH in top gear? Who tries these things? :mrgreen:

I recall all the old British mags always did a "nonsnatch" test or some such thing. A bike was a worthwhile purchase if it could smoothly pull away from almost walking speed in top gear. I don't know who actually rides like that, nor should one, however the testors always though it terribly important!
Click to expand...

I had a single carb '62 Atlas that could cruise 25 mph in top. My brothers '65 could do that also. My '63 dual carb stock out of the box, OTOH, would start to buck at 35 mph, but could beat the snot out of either. Why? ....Still a mystery to me.
 
Yes, these intakes are 1" long, and 28.5mm diameter.
So will these work with my 30mm Concentrics?

Atlas Cafe Racer Dual Carb Selection
 
You could use them as they are, but for a perfectionist like me I would blend the spaces in so that there is no step.
 
If you have been happy with Jim's PWKs on your 850 then why not get a set for the Atlas, I am sure Jim can set up a perfect match for your Atlas.

Ashley
 
mschmitz57 said:
Yes, these intakes are 1" long, and 28.5mm diameter.
So will these work with my 30mm Concentrics?
Click to expand...

Yes I don't think there is much of a problem, but the longer monobloc carb with factory air cleaner were very cramped. Therefore the shorter manifold. The 1.5" manifold with concentrics is even a bit tighter.
I found this out when injecting my 63 atlas with my 68 dunstall atlas concentric components. I still used the original factory air cleaner. I finally figured there is more than one version of the air cleaner. :roll:
 
mschmitz57 said:
I've got a `63 Atlas Cafe Racer.
I bought it with the single Amal Concentric and it's been a nice running setup but I'm beginning to think
it's a little too sedate and it might be time to install dual carbs on the old girl. I've already got 2 sets of Concentrics.
A 930 and a 932 set. The 930's are older types without the drain plug in the bottom but they are in nice shape but probably need
to be cleaned + sleeved. The 932's have been overhauled and sleeved by Lund. I was going to use them on my 850
but I ended-up the a set of PWKs instead. I bought a set of the straight Atlas intake manifolds for just such an occasion.
Will 932's work on an Atlas?

Atlas Cafe Racer Dual Carb Selection
Click to expand...

Hey Mark, what kind of padding are you using under your tank there?
 
Its just foam secured with tie-wraps.

I had a project that came with bubblewrap. !
Apart from having to renew it now and then, while it sat...
 
I originally installed some of the rubber tank buffers but I found that the tank was only supported at a couple
of points front and rear. Any additional rubber buffers did not contact the tank in the center.
I didn't like the idea of the middle of the alloy tank being unsupported when full of fuel and having to deal with the
vibrations of the rigid mounted motor, so I added some foam pipe insulation for additional support, and it also keeps
the tank more evenly centered on the frame. It's not too visible in the center as the tank sides cover it.

As far as installing PWK's on it I guess I'd rather keep this bike vintage and run period correct carbs.
Besides, I don't want to shell-out for another set of carbs when I already have a set of Amal 930's on the shelf.

Atlas Cafe Racer Dual Carb Selection


Atlas Cafe Racer Dual Carb Selection
 
30 or 32, take your pic, I'd guess 30mm on that bike.

But, whatever the carb, it must have 2.

Twin cylinder Cafe racers have twin carbs. Period.

Unless its for a girl.
 
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