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Discussion Starter #1
I was thinking about the single carb manifold I had bought years ago for my 850 Norton. Nice idle, easier starting, better gas mileage, etc. Started wondering if it could be done on a Bonnie?
All the planning went well till I looked at the pace the single carb would go. No room. Hmmmm
Later in the day while looking at a vendor booth (at the Barber Motorsport complex), I came upon an XS650 Yam done with a single carb. It too has a single backbone right in the way of a single carb.
This person had made a curved manifold that had a single offset (to the left) carb spigot. So, the single carb will live on the left side. Only grief I see is that the intake tracts are now different lengths. Nothing that can't be taken care of with a flow bench. Wondering about doing a similar gag for a carbie Thruxton.
Anyone try a single carb on a late model Bonnie/Thruxton?
thanks,
-Armen
 

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Some reading to be going on with, on the first one member locopony has actually fitted one to his America, see post 57. Work is ongoing but he seems to post a lot more on another forum. There's a lengthy thread there where he gives us the full story: http://www.bonnevilleamerica.com/forums/showflat.php?Cat=0&Number=552042&an=0&page=0#Post552042

Some threads here

http://www.triumphrat.net/twins-technical-talk/160847-single-carb-hinckley-twin.html

This thread was started by him to give us more details:

http://www.triumphrat.net/cruisers-...4-single-carbs-for-americas-and-speedies.html

http://www.triumphrat.net/twins-tec...-carb-on-bonneville-instead-of-two-carbs.html
 

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One question................ Why would you do this to such a fine machine?
I think he answered that in his original post.

Also, don't be fooled by thinking that 2 are that much better than one, performance-wise. On our bikes, only one carb is being used at a time, so a single carb can flow nearly as well as 2 (which is why I hear (don't know from personal exp) that the old Tigers were just as fast as the Bonnies until the higher speeds, when the Bonnie's took over) A single carb can continuously flow, alternating from one cylinder to the other, instead of the start-stop flow that the carbs on a twin carb model will do.

I wouldn't do it to mine, but love the idea behind it.
 

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With the old Amals that were fitted back in the day that seemed to need constant fettling and re-synchronisation I can appreciate the advantages of having to deal with only one of them to keep the bike running sweetly. The Keihin CVKs we have on our bikes don't (generally) present the same issues so I don't see a great (or any) gain by replacing them with a single carb and manifold, especially considering the work involved.
 

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LOL you guys need to stop looking at the old bikes and thinking what worked on them will work on these motors .You can treat a 4valve motor the same as the old motors.
If you want to make carbs better buy a set of fcrs and be done they will put a smile on your face.At high rpm on these motors both carbs work hard all the time.One side of the head flows about as much as both sides of the old bikes.You are allso trying to feed a much bigger cc motor.
 

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I think the point is that some folks aren't looking for performance out of a Bonneville, they want simplicity in their lives.
I know Mike, we've had this discussion before.
If it wasn't for the ergonomics of fitting the carb around the backbone, I would have tried it long ago. It eliminates the need to synchronize carbs, & hopefully better fuel milage.
I get real frustrated that my Bonnie gets @45-47 MPG (US) and my 93 Yam GTS1000A (retuned FZR motor) makes 100+ HP does 145+mph, 11.8 1/4 mile and I average 47mpg with a top of 52mpg. Oh yeah, and it weighs over 600lbs.
I understand what your saying is valid from a performance standpoint. It's just a different perspective on what you want from a motorcycle.
 

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:) Back in the day, I had a single SU on my T140. It lost nothing in performance terms, and gained hugely in economy terms. 75mpg anyone? I never wished for twin Amals at all!
 

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:) Back in the day, I had a single SU on my T140. It lost nothing in performance terms, and gained hugely in economy terms. 75mpg anyone? I never wished for twin Amals at all!
Fantastic carb. No jets as such, but an easily adjustable fuel delivery device that covered the whole range, built-in push-button device to check mixture was correct, an oil filled dashpot that acted as a device to stop throttle hesitation and flat spots on sudden throttle opening, brilliant.
 

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:) Yes, the whole assembly,carb, manifold, cables etc, was available for Bonnies and Commando's as a kit. It only took about 2 hours to fit and set up, and afterwards it never needed to be touched again! I really think that in these days of the £6 gallon, a modern SU type conversion would be a good seller.
 

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I think the point is that some folks aren't looking for performance out of a Bonneville, they want simplicity in their lives.
I know Mike, we've had this discussion before.
If it wasn't for the ergonomics of fitting the carb around the backbone, I would have tried it long ago. It eliminates the need to synchronize carbs, & hopefully better fuel milage.
I get real frustrated that my Bonnie gets @45-47 MPG (US) and my 93 Yam GTS1000A (retuned FZR motor) makes 100+ HP does 145+mph, 11.8 1/4 mile and I average 47mpg with a top of 52mpg. Oh yeah, and it weighs over 600lbs.
I understand what your saying is valid from a performance standpoint. It's just a different perspective on what you want from a motorcycle.
MY 1087 GETS UPPER 40S MPG.One carb wont do any better then 2.get some fcr carbs you wont have to work on them once there set,you will get good mpg (if you dont get wild with the right hand).
 

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Discussion Starter #13
A few things...

First of all, my bad for not adding my usual "I'm not asking for permission, only information".
Thanks Forchetto. I'm looking forward to hearing about the development of a single carb set-up.
As to the other comments, well...
The stillborn '77 Commando was to have a single carb. Testing showed very good power, MUCH better gas mileage, and less maintenance.
Folks in the Norton Club often get 50+ MPG with the single carb kits.
Nice to be able to go forever on a tank, and nice not to waste money on gas. And nice to not waste gas.
True confessions-I like to work on the bikes at least as much as I like riding.
As to the rules not applying to two valve and four valve motors-this had me laughing.
My inspiration for this whole gag was reading about the new NC700 Honda. Gee, undersquare parallel vertical twin, tuned more for torque than horsepower, etc.
Duh, they just 'invented' the 1950s-70s Brit bikes.
And this one gets 70MPG. And it uses a single throttle body for two (4 valve) cylinders. Gee, kinda like I was talking about for this project.
As to the high RPM potential. Who cares? If I want high revs, I'll ride the G/Fs 675 Street Triple. I like low end. Rarely run my Airhead past 5K RPMs.
thanks to those with useful input.
-Armen
 

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Why will a single carb' give better MPG?

I think if MPG and simplicity were my motivation I'd just go and get an EFi model or fit EFi to a carbie. Are the Bonneville carb's so bad that this mod' is going to save a great deal of trouble?
 

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On what planet is EFI simpler than a single carb?
The EFi system is simply a tube (OK 2 tubes) with a means of controlling the airflow and an electronically opperated valve to meter out the correct amount of fuel, these fuel requirements are determined by a computer program using values from a small number of sensors and a database of values held in a map each part doing its specified job and no other. Oh, and they supply their own diagnostic system.

Compare that to a carburetor which has to employ a number of circuits as no one can supply the needs across a full range of engine requirements each of these has to supply the combined functions of conditioning and delivering the fuel as well as the air (which is used in the fuel delivery process as well as having the jobe of getting itself to the cylinder), of course these circuits don't just start and end at specific conditions but instead overlap and interact so if you have an issue you have a devil of a job even finding where it is let alone solving it.

The only way in which carb's are simpler is that more people have spent more time with them and so the knowledge base is larger and so there are more people to confuse you with advice on t'Internet.
 

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single carb simple on these bikes now thats a joke.Will take more work and money then buying some good carbs and wont get 1 mpg more. with the screwed up intake you will have to make one bore will be lean and the other will be rich.You are going by other bikes that are nothing like our bonnies.But its your bike have fun.
 

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Discussion Starter #18
Reality check. Can we remember that tens of millions of cars use a single carb or single point fuel injection and they don't all blow up or run badly?
And get better MPG than our bikes. And meet tougher pollution standards?
Oh, there I go again, injecting facts into the keyboard flatulance...
 

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Armen, there's a lot of of engineering that goes into intake design.
You can't compare one design to another especially with the NC700.
This engine is far from being a vertical twin and is under square for a reason. For years manufacturers have been using variable intakes, cam timing, etc. Some use two separate intakes and MAF sensors on V designs. There is a good chance you will gain in one area and loose in another, it's about flow and filling the cylinders. Adding angles and different length intake runners can be a lesson in what I could do I shouldn't do. You are correct, your bike, no permission required.
 

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Reality check. Can we remember that tens of millions of cars use a single carb or single point fuel injection and they don't all blow up or run badly?
And get better MPG than our bikes. And meet tougher pollution standards?
Oh, there I go again, injecting facts into the keyboard flatulance...
And typically they will make less BHP per unit capacity than our (reletively low powered for a motorcycle) twins. I'm sure if you re-tuned your bike to make, for instance, 50BHP/Litre you would improve you miles per gallon althogh I suspect you'd also reduce your smiles per gallon (for me at least), they also tend to have symetrical inlet manifolds which attempt to treat all cylinders the same (equally badly).

I'm not convinced that these cars will give more MPG either, most cars I've owned (apart from a Diesel Vauxhall and a 4.2L Jaguar XJ6 with SU carbies) have returned 30MPG on average, I know things have picked up (allegedly) in recent times but that is due to direct (into the cylinder) injection system.

Things are the way they are for a reason, one carb' or TB per cylinder gives a less restrictive path to the inlet valve, putting the fuel delivery closer to the cylinder gives less opportunity for the atomisation to be lost, timing the fuel delivery to the valve (or better still ignition with direct injection), this and all the other little tricks which can be played add up to more efficiency, power and MPG both improve BUT for most of us when we have more power we tend to use it and that is what drops the MPG - Using the power not giving the engine the ability to produce it.
 
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