Carb size for Atlas

Matchless

Matchless

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I am currently building up a 1965 Atlas by using the parts I have lying around. The bike is on a Nova & I want to get it running in order to register it. The only decent pair of carbs I have spare are a pair of 32mm Concentrics which I could jet to suit. What would they work like on the Atlas which has 28mm ports & parallel bored 28mm inlet manifolds.
Any ideas?

Martyn.
 
32mm might be a bit big. The later Atlas' came with 30mm Concentrics jetted: 240 main jets, no air cleaner, 220 with, 25cc pilot, throttle slide 3, .107 needle jet.
HTH
 
All NHT semi down draft heads were 28.5 mm port and manifold from 650 and 88ss through 20M3S.
13% bigger carb is no big deal, You may notice more difference in going from low comp atlas pistons to commando pistons if that is what your build up consists of.
I don't foresee a problem if they are in fact good carbs...They may seem slightly as if they are on a quick twist throttle.
 
Thanks for the replies. I think I may as well try them & see how it runs. Maybe taper boring the inlet manifolds as per MK2a Commandos would be worthwhile?
 
Matchless said:
Thanks for the replies. I think I may as well try them & see how it runs. Maybe taper boring the inlet manifolds as per MK2a Commandos would be worthwhile?
Click to expand...

Re; “Maybe taper boring the inlet manifolds” that’s what I would do, if you want to run 32 mms,
but leave the head inlet ports stock.
 
That's what I will try. The intention with this bike once it is registered is to build a light weight bike with no lights or unnecessary parts for hammering around the hills where we now live, so I want a nice torquey motor but am unlikely to be going over the ton very often.
 
Matchless said:
That's what I will try. The intention with this bike once it is registered is to build a light weight bike with no lights or unnecessary parts for hammering around the hills where we now live, so I want a nice torquey motor but am unlikely to be going over the ton very often.
Click to expand...

Sounds fun !
 
There was a thread on this Forum that discussed the effect of a step-down bore.

IIRC, someone at BSA inadvertently fitted a large bore carb to a smaller bore inlet, and unexpectantly got an increase in HP. Anyone else remember this thread?

This might be something to research before taper boring.

I suppose the step generated turbulence that broke up and dispersed the fuel mist.

Slick

Edit: Found this .....

https://www.accessnorton.com/NortonCommando/airflow-port-taper.16013/#post-232294
 
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Hope so Nigel. The Vincent engined bike idea was just going to be TOOOO much dosh!
Thanks for that Slick. The BSA experiment was at the back of my mind so I will do a bit of reading.
 
texasSlick said:
There was a thread on this Forum that discussed the effect of a step-down bore.

IIRC, someone at BSA inadvertently fitted a large bore carb to a smaller bore inlet, and unexpectantly got an increase in HP. Anyone else remember this thread?

This might be something to research before taper boring.

I suppose the step generated turbulence that broke up and dispersed the fuel mist.

Slick

Edit: Found this .....

https://www.accessnorton.com/NortonCommando/airflow-port-taper.16013/#post-232294
Click to expand...


I do recall the story. It was told by Roland Pike, the BSA factory tuner.
It was an accidental discovery. A young apprentice picked up a " Too big" carb for a Goldstar which was being dyno tuned. No one noticed the size until after the dyno test, which showed several HP up on any previous test. When they realized the carb was too big for the inlet, a new tapered stub was made. The power disappeared.
A standard stub with step at " too big" carb was returned and the power came back.

Glen
 
I don’t doubt the “Gold Stars with the step in the inlet” story, as , surprise, surprise I have a KZ/ GPZ 305 that has a blooming big 33 bore diameter CV carbs on and there is a “bump” on the bottom on both the inlet ports- I guess I will not be removing this bump. I was considering using this method when I get my Norton back together and going on a rolling road to see if it has any benefit.
 
Matchless said:
Maybe taper boring the inlet manifolds as per MK2a Commandos would be worthwhile?
Click to expand...

I'd strongly suggest you leave well enough alone...
At the 92 National Rally there was a dyno for testing and bragging rights. My 28.5mm RH1 head was on my combat. The original combat head had been ruined by a local shop and I was going to renew the seats. The carbs and manifolds were still the 32mm combat version.
The result was that my otherwise stock combat was the highest RWHP(non race engine) @ 47 present at the meet. Leo Geoff, american drag racer, was there and claimed my bike was the highest non professionaly tuned combat commando he had ever seen. The best being 49 hp. Once I eventually put the rebuilt stock matching port combat head back on it never seemed as good.
I am going to build a combat cam atlas to exactically mimic the 28.5 mm porting with 32mm carbs & straight manifold.
 
I have been following this thread with interest as I am also doing a 1966 Atlas, my one came with a pair of 28mm concentrics which match the manifolds and ports and are in good condition. i also note with interest that the 650SS I own has 376 Monobloc Carb and a Half setup with inlet port sleeves, would the Atlas have had a similar sleeve set up? I note the '66 Atlas would have had 389/689 Carbs.
Thanks Al
 
66 seems to be the only year for 389/689 (I have a 66 and 3 other atlas') all the others have 389/chopped 389 or concentrics.
I have never seen an atlas with the port reduction sleeves, or documentation so stating .
My 68 atlas(Dunstall) has 32mm concentrics.
 
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dynodave said:
66 seems to be the only year for 389/689 (I have a 66 and 3 other atlas') all the others have 389/chopped 389 or concentrics.
I have never seen an atlas with the port reduction sleeves, or documentation so stating .
My 68 atlas(Dunstall) has 32mm concentrics.
Click to expand...
Norton fitted reduction sleeves on the single carb set up as was mine when I purchased it. some pervious owners have bored the head t 32mm dia., which is too big, and consequently fitted 28.5mm sleeves- check he port size.
 
Bernhard said:
Norton fitted reduction sleeves on the single carb set up as was mine when I purchased it. some pervious owners have bored the head t 32mm dia., which is too big, and consequently fitted 28.5mm sleeves- check he port size.
Click to expand...

What year was yours? I understand our "export" 62 had gotten a few single carb setups. The single carb do seem to be quite rare here. My 63 was dual carb and no sleeves were evident.
I never expected to see documentation for sleeves on 750, did you ever see some sleeves documented in any IPL?

My atlas was "hot rodded" before delivery by Dunstall dealership and accounts for the nonstandard parts and the ported 32mm carbs/manifolds and head. In actual fact, it is quite mellow and not even up to combat like performance. I believe most of the performance comes from the compression increase.
 
dynodave said:
What year was yours? I understand our "export" 62 had gotten a few single carb setups. The single carb do seem to be quite rare here. My 63 was dual carb and no sleeves were evident.
I never expected to see documentation for sleeves on 750, did you ever see some sleeves documented in any IPL?

My atlas was "hot rodded" before delivery by Dunstall dealership and accounts for the nonstandard parts and the ported 32mm carbs/manifolds and head. In actual fact, it is quite mellow and not even up to combat like performance. I believe most of the performance comes from the compression increase.
Click to expand...

As far as I am aware all Atlas from the factory came with the dished 7:5 -1 pistons (no longer available)- as was mine, it was an ex police 1965 with singe carb that gave a top gear performance of 15-105 MPH!
 
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