Pros and cons of different carbs? Keihin vs Mikuni vs Amals?

Nortorious

Nortorious

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I'm ready to change my carb setup and I'm looking to get the current wisdom on all the best carb setups out there along with the the good, bad, and ugly of each. I read the old threads and I didn't see any recent updates on this that compared the merits of each setup.

I'm getting my engine rebuilt and with that I want to mate it with the very best nee carb setup out there. It seems the common options are dual Amals (current setup that is failing me), single Mikuni, dual Mikunis, or the Dual Keihin Flat Slide FCR 35.

Right now I'm leaning towards the Keihins but am really not sure yet. Matt at cNw will be installing and tuning whatever I end up getting so I'm sure I'll be getting the best of whatever I choose. I just need some help figuring out the best choice. (@BrianK I wonder if you are still happy with yours)

On this particular issue I'm willing to spend whatever it takes to get the best setup with regards to great performance,.usability, and easy maintenance. I would love a "set it and forget it" carb but I know that doesn't exist. I'd like something that is as close to that as I can get though so I'm not having to clean and fiddle with my carbs a lot.

I understand the Keihins are more expensive, but cost aside do they beat out the Mikunis and Amals on all categories? Are they harder to start up and get the bike warm?

Does one of the carb setups start easier over others? And does one of the carb setups shine for delivering even power from 1000 to 7000 RPMs?

Matt is recommending the single Mikuni or dual Keihin but I see some people go with the dual Mikuni instead of single..

Am I asking the right questions?

In other words, if money is no object what's the best carb setup for a 73 Commando 850 with an owner that tries his best but isn't a carb expert?

Thanks for the advice!
 
kommando said:
Fuel injection
Click to expand...
Is this a joke or has someone actually pulled this off? I saw in old threads that it was attempted successfully.

Either way, I think FI on a Norton just feels wrong. But like so many wrong things I'd try anything once.
 
If you've read past posts you know that a single Mikuni is not going to match the top end of a pair of Amals. I've personally done side-by-side tests of two Commandos so equipped with carbs swapped between them. If the reduced top end is OK, go with whatever single carb is easiest to deal with. For dual setups, I'd want to see an objective dyno test (or side-by-side "race") showing EITHER that a pair Keihins produced more power than a pair of Amals OR produced equal power accompanied by "easier" maintenance/general function.
 
I'm very happy with my dual FCR35's on my 850.
Starts well (nearly always first kick) and runs well.
I did disable the accelerator pump (tentatively at first) and found that it isn't needed, even when going sudden WOT.
No choke, tickling or enrichment circuit, just simple and very, very good.
It's good for 124mph at 7000rpm and is a kitten around town.
Cheers
 
Ask 10 Nortoners and you shall receive (at least) 11 answers…

Here’s mine:

The best carb for a Norton is the one that’s in good condition and set up properly!

With that answered, here’s some other thoughts…



All of the options you suggest run extremely well WHEN in good condition and set up properly.

Amals look period. They’re easy to fit and look after. New Premiers are good, once QC’d by Amals QC dept (ie the customer). They run well, they flow plenty for stock ish motor, they start well. Amals are rather basic (read crude), that has a benefit in that they are way more tolerant to not being set up perfectly, to different gas and atmospheric pressures, etc. So for cost, ease of fitting, performance and looks, there’s really not a lot wrong with good ole Amals.

The two Japanese carbs mentioned however are far better made, they are far superior instruments in every aspect.

Mikunis are great. That’s why they’re fitted to literally millions of bikes.

A single Mikuni on the necessary manifold, will restrict (basically kill) performance over 5,000rpm. It’s probably the manifold rather than the carb itself which does this, but nevertheless, it does. If your riding style is to ride below this then they are a great option. Personally, I feel that sacrificing the Commandos fabulous and addictive power up to 6,000 basically defeats a large part of the objective of having a Commando !

I strongly dispute the claim that a single Mikuni runs any better than CORRECTLY set up twin carbs. So, IMHO the actual advantages to a single are low cost, and better fuel consumption.

Twin Mikunis overcome that performance limiting factor, but lose the low cost and good fuel consumption benefit. And they don’t fit nicely, they need splaying out somewhat (on splayed manifolds) as they’re too wide for the port spacings. IMHO this isn’t aesthetically pleasing. You might also have to put some time and effort into setting them up properly.

The FCRs are one of the finest carbs ever made. With the ubiquitousness of fuel injection now, they are likely to remain one of the best ever made. You only have to go back in time to days before FI ruled the race paddock, in those days nearly all the top boys ran FCRs or flat slide Mikunis.

The biggest downside to the FCR is setting them up on an unknown configuration. But that’s not an issue to Commando owners as Matt has done the leg work.

Another downside is the cost of course. And some say they look wrong, personally I think they look great.

The FCRs flow way better than the other options. The accelerator pumps help with regards to maximum acceleration, reducing the need to be precise with the twistgrip.

Starting is excellent, you simply give the twistgrip a twist which squirts some fuel in the ports which has a similar effect to flooding the Amals.

I have FCRs on mine, when I had it on the dyno a while back, the dyno man said “Thats the best fuelled carb bike I’ve ever seen” the AF ratio graph was that good! And this was a guy who normally does Japanese sport and race machines !

You simply cannot fit better instruments to a Commando. If money is no object, and you want the best, FCRs are it.
 
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Fuel injection beats playing with jets and needles, especially when combined with an engine management system. My bike has to comply with historic racing rules, if I don't want to race against moderns. The prime requirement with carburetors is to get fine enough mixture adjustment. But if you get the mixture optimised right across the whole range of throttle openings, the set-up might become temperature dependent. So most road bikes are probably jetted a bit too rich. A single Mikuni carefully tuned is probably better than two poorly tuned Amals. With Mikunis, you get more choices in jets and needles.
I use two 34mm Mk2 Amals, but I use Mikuni needles - with methanol fuel, With methanol , if it is left in the carbs - Mikunis form a white zinc oxide in the carb bowls , which can stuff things. I think Amal carbs have less zinc in their aluminium. I don't know what happens when you use petrol which contains ethanol.
Best power with petrol or methanol is achieved by jetting as close as possible to detonation, but not doing damage. If you raise the comp.ratio, or advance the ignition timing - it has a similar effect to leasning-off the jetting - you move closer to detonation and burning things.
 
Fast Eddie said:
Ask 10 Nortoners and you shall receive (at least) 11 answers…

Here’s mine:

The best carb for a Norton is the one that’s in good condition and set up properly!

With that answered, here’s some other thoughts…



All of the options you suggest run extremely well WHEN in good condition and set up properly.

Amals look period. They’re easy to fit and look after. New Premiers are good, once QC’d by Amals QC dept (ie the customer). They run well, they flow plenty for stock ish motor, they start well. Amals are rather basic (read crude), that has a benefit in that they are way more tolerant to not being set up perfectly, to different gas and atmospheric pressures, etc. So for cost, ease of fitting, performance and looks, there’s really not a lot wrong with good ole Amals.

The two Japanese carbs mentioned however are far better made, they are far superior instruments in every aspect.

Mikunis are great. That’s why they’re fitted to literally millions of bikes.

A single Mikuni on the necessary manifold, will restrict (basically kill) performance over 5,000rpm. It’s probably the manifold rather than the carb itself which does this, but nevertheless, it does. If your riding style is to ride below this then they are a great option. Personally, I feel that sacrificing the Commandos fabulous and addictive power up to 6,000 basically defeats a large part of the objective of having a Commando !

I strongly dispute the claim that a single Mikuni runs any better than CORRECTLY set up twin carbs. So, IMHO the actual advantages to a single are low cost, and better fuel consumption.

Twin Mikunis overcome that performance limiting factor, but lose the low cost and good fuel consumption benefit. And they don’t fit nicely, they need splaying out somewhat (on splayed manifolds) as they’re too wide for the port spacings. IMHO this isn’t aesthetically pleasing. You might also have to put some time and effort into setting them up properly.

The FCRs are one of the finest carbs ever made. With the ubiquitousness of fuel injection now, they are likely to remain one of the best ever made. You only have to go back in time to days before FI ruled the race paddock, in those days nearly all the top boys ran FCRs or flat slide Mikunis.

The biggest downside to the FCR is setting them up on an unknown configuration. But that’s not an issue to Commando owners as Matt has done the leg work.

Another downside is the cost of course. And some say they look wrong, personally I think they look great.

The FCRs flow way better than the other options. The accelerator pumps help with regards to maximum acceleration, reducing the need to be precise with the twistgrip.

Starting is excellent, you simply give the twistgrip a twist which squirts some fuel in the ports which has a similar effect to flooding the Amals.

I have FCRs on mine, when I had it on the dyno a while back, the dyno man said “Thats the best fuelled carb bike I’ve ever seen” the AF ratio graph was that good! And this was a guy who normally does Japanese sport and race machines !

You simply cannot fit better instruments to a Commando. If money is no object, and you want the best, FCRs are it.
Click to expand...
Amal Mk2 single or twin?
 
During WW2 the Mk9 Spitfire had a carburetor and supercharger. the Focke Wulf 190 had fuel injection and probably a blower. In a fight, the Focke Wulf 190 could just dive away, while the Spitfire left a clod of black smoke out the back as the fuel level rose. Some fighters had water injection, which would raise the comp. ratio. It also probably cooled the charge in the intake manifold due to latent heat of vaporisation.
 
You could use a mikuni TM 40mm flatslide as I have done
But you will have to cut some of the gusset away on the frame to make one fit
Or there's the SU conversion that I believe gives really good mpg at the expense of losing a little top end
But if money is no concern I'd go with the FCRs
 
acotrel said:
During WW2 the Mk9 Spitfire had a carburetor and supercharger. the Focke Wulf 190 had fuel injection and probably a blower. In a fight, the Focke Wulf 190 could just dive away, while the Spitfire left a clod of black smoke out the back as the fuel level rose. Some fighters had water injection, which would raise the comp. ratio. It also probably cooled the charge in the intake manifold due to latent heat of vaporisation.
Click to expand...
I thought Miss Shilling sorted that problem out?
 
acotrel said:
During WW2 the Mk9 Spitfire had a carburetor and supercharger. the Focke Wulf 190 had fuel injection and probably a blower.
Click to expand...

Not entirely accurate.
military-history.fandom.com

Miss Shilling's orifice

Miss Shilling's Orifice was a very simple technical device made to counter engine cut-out in early Spitfire and Hurricane fighter aeroplanes during the Battle of Britain. While it was officially called the R.A.E restrictor, it was normally referred to under various names, such as Miss Tilly's...
military-history.fandom.com military-history.fandom.com
www.calum-douglas.com

ROLLS-ROYCE MERLIN CARBURETTOR DEVELOPMENT - Calum Douglas

© Chris Starr, 2022. Preamble, by C. Douglas The story of the carburettor of the Merlin in the Second World War, more specifically the Battle of Britain, and the German fuel injected engines has been an ongoing saga since 1940. For a variety of reasons it has not been properly studied until...
www.calum-douglas.com
Therefore, Spitfires MkI and MkII during 1940 encountered flying against the Messerschmitt Bf109E and before the Spitfire MkIX or FW190 had entered service. The BMW 801 of the early FW190 had a two-stage supercharger, not a "blower" by which I assume you mean a turbocharger (as a supercharger is also known as a blower).

However, once again I would urge you to please try to stay on topic (the thread title) or you could end up 'black-flagged' again.
 
You want a constant pull from 1000 to 7000 rpm ?? How often do you take it to red line ... 6000 or even 5000 rpm . The last time my bike saw 5000 rpm was when i timed the boyer and never after that .. I installed a 36 mm Mikuni on my 75 / 850 about 16 years ago .. Set it up and forget about it . Instantly , 65 % of my electrical issues went away .

In your post you mentioned Matt's recommendation ........................ There's a reason he says that
 
Ron Hulton said:
You want a constant pull from 1000 to 7000 rpm ?? How often do you take it to red line ... 6000 or even 5000 rpm . The last time my bike saw 5000 rpm was when i timed the boyer and never after that .. I installed a 36 mm Mikuni on my 75 / 850 about 16 years ago .. Set it up and forget about it . Instantly , 65 % of my electrical issues went away .

In your post you mentioned Matt's recommendation ........................ There's a reason he says that
Click to expand...
Exactly, depends on the desired use.

What he actually wrote was “Matt is recommending the single Mikuni OR dual Keihin”

ie if you don’t intend to ride above 5k, go Mikuni… if you do, go FCR.

Mine see‘s 6k every time I ride it! I couldn’t do that when it had the Mikuni on, even at 36mm.
 
My 2 current riders have totally different set ups. My Combat is stock, with the original 932’s (sleeved), and a TriSpark. My 1970 has a single 34mm Mikuni, a Mk3 Boyer, and a 3S cam. Both run unbalanced peashooters.

1) The Amals require constant fettling to provide top performance, principally air screw o-rings and needle jets annually. When working well, the throttle action is completely smooth.

2) The Mikuni is dead smooth as well and extremely consistent. Idling is rock solid, and the bike pulls powerfully to 6500 in top gear with a 20 tooth sprocket (testing purposes only, might go to 7000, but saw no need to find out). It does not run out of power on the top end. Starting is one kick 95% of the time. A lazy kick will start it hot. I see no reason to put an e-start on this bike.

Overall - the 1970 is the quicker bike.

I am relatively new to Mikuni’s and was never a fan until I experienced a few. I think that anyone who tells you that they limit the performance of the bike needs to further refine their jetting. I have had 2 bikes go through the shop this year that came in with incorrect jetting - they both left with much better performance.

Bonus - the 1970 doesn’t smell like gas - lol!! Important if you are transporting it in a toy hauler.

FWIW
 
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