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

Fast Eddie said:

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.

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You are 100 % correct .... desired use will probably account for 75 % of ones decision making. Matt recommended Mikuni or Keihin , all i said was take note of Matts recommendation.

Here in Ontario , doing 40 kph over in a 80 kph zone will see your vehicle towed and impounded for 2 weeks , a large fine and automatic licence suspension ..50 kph over in a 100 kph zone will see the same .. vehicle towed , licence suspension , 6 demerit points , double insurance premium and a fine up to $10,000 ... Thank You Mikuni

Just for clarity I‘d like to state that ‘top end power’ is not necessarily the same as ‘top speed’.

Its seeing 6k in the lower gears that give me a thrill. I seldom see it in top.

I've run all three brands on the same Norton. Sudco Keihin 35mm FCR kits for the Norton engine work the best by a large margin if low maintenance and performance is important. The slides do rattle at idle, and some people may find that disconcerting.

My comments about starting with the FCR carburetors would be irrelevant since my Norton motor sits straight up. I have the accelerator pump functioning. The pump shot makes the motor easier to start up. How easy a motor starts would depend entirely on the owner of the bike and state of tune.

34mm Mikuni carburetors did work well enough, but they stick out kinda funny behind the motor.

The Keihin 33mm CRS carburetors are probably good low maintenance performers in that 33mm is a nice size on the street and they have chokes for easier starting. I have not used them on a Norton though, so I suppose that would be irrelevant. Note: You can race in vintage classes with CRS carburetors, but not the FCRs because of the accelerator pump.

Not a fan of Amals having dealt with the early 930's in the 70's. Making them work for a while was not a challenge. I never cared that much for the o-ring flange design or the soft material they were manufactured from.

I don't think any of those carburetors let me start the motor and not have to hold the throttle open a little to warm up the motor and get a smooth idle. However, I don't tune carburetors for start up and instant cold idle. I tune for a warmed up motor. If you want instant cold idle fuel injection would be the ticket. Very impractical unless you are a master mechanic and have the time to cobble something together.

I do have some prejudice and won't run a single carburetor on a twin. Lackluster performance on the one Norton Commando I had with a single on it.

I may be off topic. I've already forgotten what the question was.

Derek Wilson said:

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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Interesting what you say about jetting on the mikuni
I've had a single 32mm CV on mine that ran really well but would cut momentarily if held for long periods above 90/95 mph I'd close the throttle and let the speed drop them open it up until it happened again
The same thing happened with a single VM 34
And again with a VM 36
This was with both taps open so I'm guessing the float needle jet just wasn't big enough?
I get none of these problems with the TM 40
The TM is not as good at starting as a pair of amals but with an electric start it doesn't matter to me
If it's really cold I pump the throttle 2 or 3 times to prime it and away it goes

Nortorious said:

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!

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What do you intend to do with your commando? maybe not quite the right question, but, how do you ride? like you stole it? gentle pottering around on a Sunday morning? Touring the continent? Trying to keep up with the spaceships and transformers that are modern bikes? or just like blasting past all the rightous bros on Hardly Dangerouses. Do you like fiddling with cables and the like on a regular basis?
You say you are getting the engine rebuilt. To a standard-ish spec? or something else? the engine is complete package from the airbox inlet to the end of the exhaust so if you have put a load of hop up bits in the engine (hi comp pistons, wild cam, gas flowed etc) it would make little sense to fit a single carb. That said, i know of one well known 'norton expert' who fitted a mk3 with a PW3 cam and also a single Mikuni....
If wanting a user friendly machine i would go with a single carb every time. Amal, Mikuni, or if you can find one an SU, easiest carb to set up by far and the most economic. Performance wise a single carbed bike will still do 100mph

Ron Hulton said:

Here in Ontario , doing 40 kph over in a 80 kph zone will see your vehicle towed and impounded for 2 weeks , a large fine and automatic licence suspension ..50 kph over in a 100 kph zone will see the same .. vehicle towed , licence suspension , 6 demerit points , double insurance premium and a fine up to $10,000 ... Thank You Mikuni

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Guess something's got to pay for universal health care

Back on topic, another con on any aftermarket carb is you will need to relocate the ignition switch, but IMO worth the trouble.

Regarding FI, that's arguably the best setup, but you need a high pressure fuel pump with return line, computer, air flow sensor, A/F sensor(s), crank angle sensor, and maybe a couple others. Then you will need to input a fuel map that takes into account all the sensor variables which generally requires a dyno. Too much trouble

Derek Wilson said:

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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Maybe it’s different for 750s? I’ve never owned a 750, perhaps the CFM requirements of a 750 are not constrained by the single Mik set up?

They definitely are on an 850 though. Don’t get me wrong, my 850 would rev beyond 5k without issue, it just wasn’t producing much extra power (increased revs don’t automatically mean increased power), so rather than chasing diminishing power returns, max acceleration was achieved by changing up around 5k and using the great low-mid range torque.

I can’t recall the thread now, but it was in a thread some time ago that Comnoz explained it is the manifold that is the bottleneck in the system for Mik set up.

If one were to remove the gusset plate on the frame and fabricate a longer manifold, this could be addressed. A big job though.

I’ve told the story on here before of a 650 Triumph race bike I rode once that had an SU on and it flew! But it has a very unique manifold and a head carefully ported to suit it.

dobba99 said:

how do you ride? like you stole it? gentle pottering around on a Sunday morning? Touring the continent?

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It's fair to say I ride it like a stole it. I get close to the redline every time it leaves the garage. I think I would feel some disappointment of the power dried up at high RPM. It is good to know the single carbs setup can still do the ton though.

After hearing from the group here and talking with Matt I'm strongly leaning towards the Keihins. I'm getting the engine rebuilt and tuned, and while the Mikuni sounds easy to manage I can't help but want that bit of extra power since I'm updating the bike.

I didn't hear anyone say the the Mikuni FCRs were any more difficult to use (once squeezed into place). Is that accurate? For those with experience, do you find that the Keihins stay dialed in once you set them correctly? Are the easier / lower maintenance than the Mikuni, the same, or worse?

Thanks for all the great information!

Fast Eddie said:

Just for clarity I‘d like to state that ‘top end power’ is not necessarily the same as ‘top speed’.

Its seeing 6k in the lower gears that give me a thrill. I seldom see it in top.

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That may be because you are concentrating on something else!!

Nortorious said:

It's fair to say I ride it like a stole it. I get close to the redline every time it leaves the garage. I think I would feel some disappointment of the power dried up at high RPM. It is good to know the single carbs setup can still do the ton though.

After hearing from the group here and talking with Matt I'm strongly leaning towards the Keihins. I'm getting the engine rebuilt and tuned, and while the Mikuni sounds easy to manage I can't help but want that bit of extra power since I'm updating the bike.

I didn't hear anyone say the the Mikuni FCRs were any more difficult to use (once squeezed into place). Is that accurate? For those with experience, do you find that the Keihins stay dialed in once you set them correctly? Are the easier / lower maintenance than the Mikuni, the same, or worse?

Thanks for all the great information!

Click to expand...

The Japanese carbs are more difficult to set up right, but once there, they do stay in tune well.

Obviously carb synchronisation is better with them, with the single Mik there is no synch! With the FCRs they are mechanically synchronised.

The only maintenance issues I’ve had with the FCRs is that the needles appear to require fairly frequent replacement. The needles take on the appearance that they have been sand blasted, I put that down the the way the slides can rattle around, especially on an old Norton twin, but am only guessing.

The only failure I’ve had is that a float housing o ring perished, causing flooding. I assume this is down to ethanol in the fuel even though I always use ‘Super’ it was after around 7 years though.

So I guess I’m saying that new needles and o rings might be required once every few years.

I believe the Mikuni is fairly maintenance free, but if left standing, the pilot jet can clog.

Both are far more sensitive to dirt than Amals and NEED decent in line filters IMHO.

Schwany said:

...Sudco Keihin 35mm FCR kits for the Norton engine work the best by a large margin if low maintenance and performance is important. The slides do rattle at idle, and some people may find that disconcerting.

...

The Keihin 33mm CRS carburetors are probably good low maintenance performers in that 33mm is a nice size on the street and they have chokes for easier starting. ...

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Someone brought their bike out to me because they thought it was "pinging" from too lean a fuel mix. It turned out to be rattling flat slides of the 35 FCRs which sounds exactly like "pinging". Otherwise they run great but a little big for most Norts. Rattling is not a problem with the 33mm CRS smoothbores but the throttle wheel and cable arrangement must be modified to clear the gas tank. Dual Mics won''t fit without being splayed to each side. A single carb will dog you down over 5000 RPM.

Nortorious said:

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!

Click to expand...

ANY carb will need cleaning if you fail at storing. Sta-Bil just stored my 100hp outboard motor for SIX YEARS, it started and ran AS NEW.
A lifelong mechanic, I feel the Amal Premiers are a great choice.
JMWO

Ron Hulton said:

You are 100 % correct .... desired use will probably account for 75 % of ones decision making. Matt recommended Mikuni or Keihin , all i said was take note of Matts recommendation.

Here in Ontario , doing 40 kph over in a 80 kph zone will see your vehicle towed and impounded for 2 weeks , a large fine and automatic licence suspension ..50 kph over in a 100 kph zone will see the same .. vehicle towed , licence suspension , 6 demerit points , double insurance premium and a fine up to $10,000 ... Thank You Mikuni

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I do like the fine schedule posted, like the drive-through at Mickey D's....

i run amals only until they wear out. then i fit VM mikunis, if they will fit. very easy to tune for the street. but theyre big. my go-to carburetter. the TM flat slide mikunis have better throttle response.

my 650 triumph non-street bike with 35mm FCRs goes 135mph. good carbs, also easy to tune, excellent for top end. but even bigger than mikunis. ive never noticed a rattle from the slides at any rpm.

Nortorious said:

I didn't hear anyone say the the Mikuni FCRs were any more difficult to use (once squeezed into place). Is that accurate? For those with experience, do you find that the Keihins stay dialed in once you set them correctly? Are the easier / lower maintenance than the Mikuni, the same, or worse?

Thanks for all the great information!

Click to expand...

What is difficult for one man or woman is I can do that in my sleep for another.

The Keihin FCRs have been set and forget for me so far. I can see wear starting on the needles as mentioned, but no issues yet. No problems with the float needle housing, but I do put some additives in my fuel and do a non-ethnol/ethonol blend on occasion.

I used Mikuni 34mm VMs in a dual carburetor setup, so don't know how the Mikuni TM flat slides would or could work in a dual carburetor configuration in a Commando frame. Once tuned/synced the 34mm VMs worked really well, and I did not have to do anything special to keep them working. Ditto on not letting the fuel dry up in them though. Especially for over 25 years. he he There is a difference in performance with the FCRs, but it is kind of subtle until getting up around 7K RPM. No regrets on getting the FCRs. They are excellent very tunable carburetors, although I really did not need them.

Some fine tuning may be required, but the FCRs I got worked as delivered for the most part on a modified 750 Norton engine. I didn't buy them from Matt, because I had to modify the intake manifolds and didn't want to mess up the nice ones that come with his kit.

Jim Schmidt's Keihin 33mm CRS solution is probably a good one. Needles would last longer and the round slides don't rattle like the flat slides on rollers do in an FCR.

The biggest issue I feel with dual Amals is synchroniztion.
Dual cables with individual idle controls.
A little difficult to maintain and really easy to mess up.
The Pender gantry goes a long ways to resolve this.

The FCR's and CRS's have the common throttle shaft to keep in sync.

One of the downsides with the FCR's here in SoCal is some of the freeway's are like washboards.
With the modulations, it's difficult to keep a steady right hand and with the quick throttle and accelerator pump, it amplyfies the issue.

Nortorious said:

It's fair to say I ride it like a stole it. I get close to the redline every time it leaves the garage. I think I would feel some disappointment of the power dried up at high RPM. It is good to know the single carbs setup can still do the ton though.

After hearing from the group here and talking with Matt I'm strongly leaning towards the Keihins. I'm getting the engine rebuilt and tuned, and while the Mikuni sounds easy to manage I can't help but want that bit of extra power since I'm updating the bike.

I didn't hear anyone say the the Mikuni FCRs were any more difficult to use (once squeezed into place). Is that accurate? For those with experience, do you find that the Keihins stay dialed in once you set them correctly? Are the easier / lower maintenance than the Mikuni, the same, or worse?

Thanks for all the great information!

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So do I.

I run a Comstock flow ported FA head with larger valves, Webcam 312A cam with radiused lifters, 9.5:1 pistons, and a Maney exhaust. No slouch of a bike, really.

I also have a modern Ducati, FWIW.

When I was redoing my bike and thinking of carbs, I had several conversations with Jim and discussed FCRs vs. Premiers and making power and flow. His opinion was given the spec of my motor, the Amals would flow fine and provide as much power as the FCRs. And they were cheaper. So I went that route.

Starting has never been an issue. Performance has never been an issue. Staying in tune has never been an issue. Obviously, I have a non-standard spec powerplant, so getting them to where I wanted them was trial and error, but that would have been the case regardless of carb choice.

The only thing about them though is what others have noted re. the soft metal - that I have had an issue with in ham-fistedly warping the bowls and having to fettle them back to flush. But, I had the carbs/bowls off regularly as I made changes to the bike,

That, and the stinky finger.

gortnipper said:

So do I.

I run a Comstock flow ported FA head with larger valves, Webcam 312A with radiused cams, 9.5:1 pistons, and a Maney exhaust. No slouch of a bike, really.

I also have a modern Ducati, FWIW.

When I was redoing my bike and thinking of carbs, I had several conversations with Jim and discussed FCRs vs. Premiers and making power and flow. His opinion was given the spec of my motor, the Amals would flow fine and provide as much power as the FCRs. And they were cheaper. So I went that route.

Starting has never been an issue. Performance has never been an issue. Staying in tune has never been an issue. Obviously, I have a non-standard spec powerplant, so getting them to where I wanted them was trial and error, but that would have been the case regardless of carb choice.

The only thing about them though is what others have noted re. the soft metal - that I have had an issue with in ham-fistedly warping the bowls and having to fettle them back to flush. But, I had the carbs/bowls off regularly as I made changes to the bike,

That, and the stinky finger.

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This is really helpful. I've been a little less talented with getting the dual Amals tuned and synched but I've gotten better at it. I also had a modern Triumph Thruxton 1200 R until a few weeks ago for when I wanted a bit more reliability and modern handling, but I sold it a few weeks ago to fund my Norton rebuild habit. I figure I can get another modern bike down the line but I'm working on getting this Norton dialed in because, as others said, there's something irreplaceable about the uniqueness of this bike. I'm going to rebuild it while there are still legendary folks like Matt willing to work on it and good people here on the forum to help.

Nortorious said:

This is really helpful. I've been a little less talented with getting the dual Amals tuned and synched but I've gotten better at it. I also had a modern Triumph Thruxton 1200 R until a few weeks ago for when I wanted a bit more reliability and modern handling, but I sold it a few weeks ago to fund my Norton rebuild habit. I figure I can get another modern bike down the line but I'm working on getting this Norton dialed in because, as others said, there's something irreplaceable about the uniqueness of this bike. I'm going to rebuild it while there are still legendary folks like Matt willing to work on it and good people here on the forum to help.

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Stock Amals or Premiers? The new ones are soooo much easier to keep right. Not much to do, really. At least for me.

Tuning is an acquired skill/habit. And it is easier with the stock exhaust than a 2-1 for setting idle/off idle up evenly since you can feel/hear the pressure differences at the silencer end more easily and the L/R differences in a Maney pipe are larger due to the header shapes.

What is the rest of your motor build spec like?

If you have a H.C. 920 with a stage 2 head like Fast Eddie - you may be better off with the FCRs. But I know he as put at least as much time into them as I have my Premiers. ;-)

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.

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Hmmm?
im not sure of the validity of this post. Negative g force is rarely an issue when ride my Commando Whether I have a Fockewulf on my tail or not. I’m sure Mrs Shilling had a most beautiful orifice and is a thing of wonder to behold but I don’t think my Amals need to be so equipped.
yours in humour
Alan