Best alternative carburetor setup?

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I discovered that NICHE sells the slide + UFO already installed for approx $65. So I installed one yesterday (4-4-2021) in my Atlas. The instructions say to reduce the pilot jet by 50% so I went down to a #20 pilot. Five kicks later it fired up (with an already warm motor) and I idled it down to where it should be. Then out for the test ride from idle to 4000 on city streets, not the best for testing. It performed very well with a noticeable improvement in that entire range. I'm sure there will be some fine tuning of the air screw or pilot jet but so far so good. I believe 4000rpm is no more than 1/4 throttle, perhaps less. I will test it at 1/2 throttle briefly down the freeway but at that opening I will be going 80+ in third gear and I think the motor will still be catching up to the throttle position. Above that it's difficult to jet for because I'm never there. Theoretical question: rev limits no concern, unlimited straight freeway with zero traffic...perfect conditions. perhaps 20 t sprocket. If the throttle is left fully on what will the eventual rpm be and approx speed?
Possible trick question. The motor would fail in a big way long before the motor caught up to the throttle opening. So I'm saying jetting for anything above half throttle is progressively more difficult to do. A rev limit is the final decider. Perhaps no one knows what the final mainjet is exactly.
Sorry, went off on a tangent.
 
And furthermore...Thunder products sells an additional product of interest...they call it a "quad flow torque wing". Looks very interesting and perhaps could be modified for amal carbs. I believe this would work best with dual carbs but has anyone tried it with a single Mikuni VM?
 
What about the recent discussions of the SU carb fitment to Nortons? Seems there used to be a kit offered and Madass140 was making inquiries (possible new kit to come?!?!?)
 
Unfortunately, carbs are viewed as black magic.
You want 55-60 mpg? You'll need to drive like Ned Tweedy, the Amals will do it.
You want the top end power the bike was designed with? The original twin Amals will do it, but you're gonna burn some fuel in the process.
There is no silver bullet.
A pair of PROPERLY set up, good condition Amals will do what the engine is capable of.
 
alan hodge said:
i just stumbled across this for my 34mm mikuni a cheap experiment and will put unifilter sock on end
www.ebay.com

Motorcycle 55mm Air Filter Velocity Stacks Wind Cup Horn Fit 32/34mm Carb | eBay

Find many great new & used options and get the best deals for Motorcycle 55mm Air Filter Velocity Stacks Wind Cup Horn Fit 32/34mm Carb at the best online prices at eBay! Free shipping for many products!
www.ebay.com
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That’s what I had in mind also , guess great minds .... , eh Alan
 
Tornado said:
What about the recent discussions of the SU carb fitment to Nortons? Seems there used to be a kit offered and Madass140 was making inquiries (possible new kit to come?!?!?)
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I've used the SU on my 850 & it's great if you don't want the top end performance. The biggest problem is it's a tight fit & the manifold is made to take this into account, which is more than likely why the top end is missing. On the plus side it will achieve 60+ mpg. I've often wondered if the Screamin' Eagle 44mm Keihin CV as fitted to Hardly Dangerous Sportsters would work instead.
 
Matchless said:
I've used the SU on my 850 & it's great if you don't want the top end performance. The biggest problem is it's a tight fit & the manifold is made to take this into account, which is more than likely why the top end is missing. On the plus side it will achieve 60+ mpg. I've often wondered if the Screamin' Eagle 44mm Keihin CV as fitted to Hardly Dangerous Sportsters would work instead.
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It’s definitely the manifold that’s the issue. the gusset plate would need removing to allow space for an appropriate manifold I believe.

Dave Degens once built a 650 Triumph for a bit of PR, the idea was it’d do 100mph and 100mpg (not at the same time of course).

We took it along to a Mallory Park test day and I was tasked with shaking it down. I was younger and stupider then and got a bit carried away, the thing absolutely flew and I was revving it to 9,000 rpm!

The point is... it had an SU carb. But it was on a fairly long Y shaped fabricated manifold.
 
Fast Eddie said:
It’s definitely the manifold that’s the issue. the gusset plate would need removing to allow space for an appropriate manifold I believe.

Dave Degens once built a 650 Triumph for a bit of PR, the idea was it’d do 100mph and 100mpg (not at the same time of course).

We took it along to a Mallory Park test day and I was tasked with shaking it down. I was younger and stupider then and got a bit carried away, the thing absolutely flew and I was revving it to 9,000 rpm!

The point is... it had an SU carb. But it was on a fairly long Y shaped fabricated manifold.
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That's very interesting as I have been wondering about trying the SU on my Atlas Domiracer. Once I've finished fine tuning the MK2s I might knock up a manifold & give it a try, as there is plenty of room between the motor & cental oil tank.
 
Fast Eddie said:
It’s definitely the manifold that’s the issue. the gusset plate would need removing to allow space for an appropriate manifold I believe.

Dave Degens once built a 650 Triumph for a bit of PR, the idea was it’d do 100mph and 100mpg (not at the same time of course).

We took it along to a Mallory Park test day and I was tasked with shaking it down. I was younger and stupider then and got a bit carried away, the thing absolutely flew and I was revving it to 9,000 rpm!

The point is... it had an SU carb. But it was on a fairly long Y shaped fabricated manifold.
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Mr Hooper's Phoenix kit needed no frame mods (not on the Mk3, at least)... It IS a tight fit, mind. Top end is still there, you just have to wait longer :)
Flexibility at lower revs seems way more 'user friendly', and a 'progressive' choke, too...
So no, not one for racing, but top marks elsewhere... IMHO
 
EstuaryBoy said:
Mr Hooper's Phoenix kit needed no frame mods (not on the Mk3, at least)... It IS a tight fit, mind. Top end is still there, you just have to wait longer :)
Flexibility at lower revs seems way more 'user friendly', and a 'progressive' choke, too...
So no, not one for racing, but top marks elsewhere... IMHO
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No arguments with any of that. I just believe that with a better manifold it’d have better WOT performance.
 
72Fastback said:
........
I have NEVER ridden a Commando with sorted Amals, so if I don't know what the top end rush feels like, maybe I won't even notice that its not there with the carb setup I get, unless it is really THAT bad. Theres always brand new 30mm Amals for $330.
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32mm Amals were stock on a Combat. I have had single and dual Mikuni setups on my bike over the years but now back to my old Amals, sleeved by Bruce Chessell. I would go for a new set of 32mm Amal Premiers nowadays.
Russ
 
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Fast Eddie said:
No arguments with any of that. I just believe that with a better manifold it’d have better WOT performance.
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Could be (and sign me up when it's available).... plus with the plethora (good word, no?) of tuning articles for the SU given it's history in four wheeled competition (internal flowing especially), it could be a tinkerer's paradise, though not as easy compared to what else is out there..
Cue Dobba99......... :)
 
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I am surprised that the larger size of the carb makes a difference to the top end, it you change the main and needle jets to suit the different vacuum - as you would have done with a smaller carb.
What revs and gearing are you talking about ? If you change the size of the carb, you might shift the torque curve up or down a bit. One thing I have carefully avoided is enlarging the ports in the head of my 850. All it has is a taper for the first 25mm to blend in for my 34mm Amal Mk2s. My motor pulls hard from bottom and would go straight through the top if I let it. But I use methanol fuel, so the jetting is never wrong - I have much more leeway than you have with petrol.
 
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alan hodge said:
i just stumbled across this for my 34mm mikuni a cheap experiment and will put unifilter sock on end
www.ebay.com

Motorcycle 55mm Air Filter Velocity Stacks Wind Cup Horn Fit 32/34mm Carb | eBay

Find many great new & used options and get the best deals for Motorcycle 55mm Air Filter Velocity Stacks Wind Cup Horn Fit 32/34mm Carb at the best online prices at eBay! Free shipping for many products!
www.ebay.com
Click to expand...
It probably does nothing useful! I use Mikuni's own velocity stack, and it has a different design feeding to the internal carb bore size not the original trumpet size which gives you a stepped inlet, however:

To be honest I use them to achieve the overall carb to inlet valve tract length I want rather than specifically improve airflow!

One of the main suppliers of Mikuni kit in the UK is adamant that he has never seen any improvement on a dyno with any design of velocity stack on a Mikuni!

This is a guy who will supply UFOs only if you supply the carb to him for modification and fitting! He knows a fair bit about them!
 
Do you guys use the specification jetting regardless of the size carb you are using ? A bigger carb means less vacuum, and the torque profile of the motor changes with the size of the inlet ports.
 
SteveA said:
It probably does nothing useful! I use Mikuni's own velocity stack, and it has a different design feeding to the internal carb bore size not the original trumpet size which gives you a stepped inlet, however:

To be honest I use them to achieve the overall carb to inlet valve tract length I want rather than specifically improve airflow!

One of the main suppliers of Mikuni kit in the UK is adamant that he has never seen any improvement on a dyno with any design of velocity stack on a Mikuni!

This is a guy who will supply UFOs only if you supply the carb to him for modification and fitting! He knows a fair bit about them!
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Tuned exhausts and inlet tracts are only optimal at one rev level. I have trumpets on my 34mm MK2 Amals because they look good. I believe the perfect length for the inlet tract in inches from opening to centre of the combustion chamber is 28,000 divided by the revs. But it might be bullshit
 
The only high quality twin carb that fits parallel on a Norton head and offers high performance is the Keihin. 35mm FCR or 33mm CRS (also in 31mm). The CRS needs alternate cable mounts. There are lower quality carbs and carbs that don't fit right and there are single carbs that don't flow well. The only other one that comes close is the Mikuni constant velocity but its sluggish performer.

Generally speaking - most quality twin carbs are just too big to fit well behind a Norton. Its a shame.
 
alan hodge said:
i just stumbled across this for my 34mm mikuni a cheap experiment and will put unifilter sock on end
www.ebay.com

Motorcycle 55mm Air Filter Velocity Stacks Wind Cup Horn Fit 32/34mm Carb | eBay

Find many great new & used options and get the best deals for Motorcycle 55mm Air Filter Velocity Stacks Wind Cup Horn Fit 32/34mm Carb at the best online prices at eBay! Free shipping for many products!
www.ebay.com
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this wont fit 34MM mikuni!!!!!!!! it came yesterday and is too small around throat diameter by about 1mm
 
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