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

MexicoMike said:
To me there are three components that produce a major part of the "look" of the classic Norton Commando. The Amals, the ham can filter, and the pair of ignition coils. I can't deal with the loss of any of those items. I have replaced carbs on just about every vehicle I have ever owned but none of them were visible OR, if they were, they had no "iconic" look to them. I put flat slides on my Ducati 900SS back in the 90s with no hesitation at all. They worked well. I didn't like the clatter but It kind of went with the jingling/rattling of the dry clutch!

Re twins/vs single - as I said in an earlier post, a few years ago we did a backroad test of two 850 Commando side by side, one with a single Mik (though I don't recall what size) and the other with the standard twin Concentrics. Up to around 4500 RPM there wasn't much difference but beyond that the twin carbs pulled away. We swapped carb setups on the two bikes and the same thing happened - the twin carb began pulling away around 4500 RPM.
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The trouble with doing that is, when you jet the carbs you would need to do it systematically to get the best out of both set-ups The vacuum characteristic is different in each case. With a single carb you have more vacuum, so the needle has to be different. The other thing is gearing. - with a single carb, you probably get less top end power and slightly more torque, so you might need to go up a bit. There is a thing about torque - if you improve it, you don't usually know what you have achieved, until you raise the gearing, and the bike goes faster.
 
If I had a road going Commando, I would either keep it a close to standard as possible, or I would go the whole hog and do everything to it. Anything in-between is probably rubbish. As standard Commando is a beautiful bike, but so is a Seeley Commando 850. I would hate to ride my Seeley on a public road - it would frustrate the hell out of me. But a nice standard 850 would be good. The closer to original a classic bike is, the more valuable it is.
 
Hello, the topic is pros and cons of different carburetors…….OK.

Here are my 2 cents :
I have experience with Monoblocs, Concentrics MK1 and MK2 and Mikunis on Nortons, BSAs and Triumphs.
The Amals are simple and work great! No doubt!

But:
In my eyes the problem with MK1 and MK2 is the material……they do not last very long. Me thinking appr. 25k km. Then you have to exchange the carburetor bodies at least.
Mk1: they got a flange to connect to the manifold …..when you are new to these carburetors and you tighten the nuts as you would like to do for this size of bolts n nuts you will for sure distort the body.
I have seen this very often!

The only problem is not that they can suck in some air cause of the distortion….the problem can be a sticking throttle valve.
No fun at all at WOT – and maybe a curve in front of you…..Good luck and hopefully some guardian angels with you!
Roundsliders with flanges in general can get this problem.

I like rubber sleeve manifolds much more like they are on the Mk2s….
Easier to put carburetors on and off…..no distortion of the body….and a better heat insulating.

IMO the MK1.5 Concentric was a good solution. Appearance from the MK1 with a rubber sleeve manifold like MK2. Don`t know why they don`t produce thema anymore…..?

Mikuni VM: I put some on Triumphs. They got for sure better material than Amals…easy to adjust and stay in the adjustment. They also have rubber sleeve manifolds like Amal MK2.

Keihin CR: for sure the Real MCoy when you want to go with roundsliders.

After several Amal roundsliders I decided to try other stuff….and it should be a flatslider.
Keihin FCR35s were too expensive and IMO looking a tad too modern.
After suggestion from my carburetor dealer in my hometown I bought 2 Mikuni RS34…flatsliders with accelerator pumps……both slides are operated with one shaft. (As Keihin FCR and CR)
(for sure boring for some members here to read my story again……sorry guys, but I love my RS!)

They always work very well….no loss of power in any rpm….and starts also very well.
After 10years and doing plenty of km I did a service this year......renewing float needles and …e pasta!
I have to say that the RS were not plug n play….I modified them from push-pull to only pull operated carburetors like Amals with just 1 throttle cable.

My conclusio:
every carburetor type got pros and cons…..as mentioned from other members before: depends on how you want to use and service your bike.

Doing not that much km and want to stay original in the appearance: go with Amal. I prefer MK2 cause of the rubber sleeve manifolds

Want a more economic carburetor: go with Mikuni VM

Want ride much km and perhaps want hard riding:

go with flatsliders…Keihin FCR or Mikuni RS

Regards, chris
 
Fast Eddie said:
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.
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I think Ron stated it best - it really depends on your expectations of performance.
I look for easy starting, smooth throttle response, smooth transitions, good mid-range performance and reliability. I ride secondary roads almost exclusively at speeds below 75 mph.

I setup a VM36 on an 850 a few months back - it pulled well to 80-90 mph. I saw no reason to rev it right out - no reason around here to go faster than that. IMHO 850’s are all about mid-range. They typically get asthmatic at about 5500 in top gear. But they can pull taller gearing if desired.

FWIW…
 
Fast Eddie said:
Blimey… most of what you guys consider icon I’ve binned !
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With 4 bikes, I dropped the 'iconic' part in the bin off of the 4th this year, since my preference is for the functional!!

Not going anywhere near FCRs......one I can't afford them, two, they are not eligible for classic racing.
 
Derek Wilson said:
I think Ron stated it best - it really depends on your expectations of performance.
I look for easy starting, smooth throttle response, smooth transitions, good mid-range performance and reliability. I ride secondary roads almost exclusively at speeds below 75 mph.

I setup a VM36 on an 850 a few months back - it pulled well to 80-90 mph. I saw no reason to rev it right out - no reason around here to go faster than that. IMHO 850’s are all about mid-range. They typically get asthmatic at about 5500 in top gear. But they can pull taller gearing if desired.

FWIW…
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What size air jet did you use? ;)
 
My basic "boy racer" attitude does not allow me to do anything to a vehicle that might reduce performance...even if I never actually use the OEM performance.

But I suppose there are folks who would remove the two four-barrel carburetors from a 1970 Hemi Barracuda and stick a two barrel on there for better mileage and easier tuneups! :eek:
 
Derek Wilson said:
I think Ron stated it best - it really depends on your expectations of performance.
I look for easy starting, smooth throttle response, smooth transitions, good mid-range performance and reliability. I ride secondary roads almost exclusively at speeds below 75 mph.

I setup a VM36 on an 850 a few months back - it pulled well to 80-90 mph. I saw no reason to rev it right out - no reason around here to go faster than that. IMHO 850’s are all about mid-range. They typically get asthmatic at about 5500 in top gear. But they can pull taller gearing if desired.

FWIW…
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Absolutely agree, depends very much on expectations AND requirements.

The Commando can be anything from a back lane torquester to a full on hot rod.

Different horses for different courses.

Back lane torquester pilots would be wasting their money on FCRs… just as hot rodders would be disappointed with a (normal kit) single carb.
 
Fast Eddie said:
But it’s only by comparing that we can put any qualitative value on such discussions.

As I’ve already said, with a single Mik, mine would rev (romp?) past 5. But COMPARED TO twin carbs, the power produced was significantly less.

Increased revs do not automatically equal increased power.

A friend of mine with a quite stock 850 has experimented with stock carbs, a single mk2 and a single Mik.

He was very happy with the single Amal until he re-tried the twin carbs and said he’d forgotten what he’d lost!

But, as the old adage goes, YMMV…
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Struth
"While I might agree that a correctly jetted single Mikuni may outperform a set of stock, poorly jetted Amals,
I would not agree that a single Mikuni on a MAP manifold will outperform a pair of correctly jetted Amals at any rpm. I have proved it several times on my dyno. Jim"

Somewhere else there is mention of dyno results in a 2014 thread. This popped up yesterday so I reread it but can't find it this morning!
Anyway the numbers listed -

Single Mikuni 35-37 rwhp

Stock 850 with twin Amala 45-48 rwhp.

That's too much performance to throw away, even in my old age!

Same dyno and same dyno operator , Comnoz
 
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Chris Zet said:
Hello, the topic is pros and cons of different carburetors…….OK.

Here are my 2 cents :
I have experience with Monoblocs, Concentrics MK1 and MK2 and Mikunis on Nortons, BSAs and Triumphs.
The Amals are simple and work great! No doubt!

But:
In my eyes the problem with MK1 and MK2 is the material……they do not last very long. Me thinking appr. 25k km. Then you have to exchange the carburetor bodies at least.
Mk1: they got a flange to connect to the manifold …..when you are new to these carburetors and you tighten the nuts as you would like to do for this size of bolts n nuts you will for sure distort the body.
I have seen this very often!

The only problem is not that they can suck in some air cause of the distortion….the problem can be a sticking throttle valve.
No fun at all at WOT – and maybe a curve in front of you…..Good luck and hopefully some guardian angels with you!
Roundsliders with flanges in general can get this problem.

I like rubber sleeve manifolds much more like they are on the Mk2s….
Easier to put carburetors on and off…..no distortion of the body….and a better heat insulating.

IMO the MK1.5 Concentric was a good solution. Appearance from the MK1 with a rubber sleeve manifold like MK2. Don`t know why they don`t produce thema anymore…..?

Mikuni VM: I put some on Triumphs. They got for sure better material than Amals…easy to adjust and stay in the adjustment. They also have rubber sleeve manifolds like Amal MK2.

Keihin CR: for sure the Real MCoy when you want to go with roundsliders.

After several Amal roundsliders I decided to try other stuff….and it should be a flatslider.
Keihin FCR35s were too expensive and IMO looking a tad too modern.
After suggestion from my carburetor dealer in my hometown I bought 2 Mikuni RS34…flatsliders with accelerator pumps……both slides are operated with one shaft. (As Keihin FCR and CR)
(for sure boring for some members here to read my story again……sorry guys, but I love my RS!)

They always work very well….no loss of power in any rpm….and starts also very well.
After 10years and doing plenty of km I did a service this year......renewing float needles and …e pasta!
I have to say that the RS were not plug n play….I modified them from push-pull to only pull operated carburetors like Amals with just 1 throttle cable.

My conclusio:
every carburetor type got pros and cons…..as mentioned from other members before: depends on how you want to use and service your bike.

Doing not that much km and want to stay original in the appearance: go with Amal. I prefer MK2 cause of the rubber sleeve manifolds

Want a more economic carburetor: go with Mikuni VM

Want ride much km and perhaps want hard riding:

go with flatsliders…Keihin FCR or Mikuni RS

Regards, chris
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Chris, the wear problem with Amals seems to be a thing of the past.
Here is a 2017 photo of the Premiere slides on my Rapide after 15,000 miles or about 23,000 Kms
I now have close to 40,000 kms on the carbs and things still look as in the photo

Screenshot_20220810-062820.png
 
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worntorn said:
Struth
"While I might agree that a correctly jetted single Mikuni may outperform a set of stock, poorly jetted Amals,
I would not agree that a single Mikuni on a MAP manifold will outperform a pair of correctly jetted Amals at any rpm. I have proved it several times on my dyno. Jim"

Somewhere else there is mention of dyno results in a 2014 thread. This popped up yesterday so I reread it but can't find it this morning!
Anyway the numbers listed -

Single Mikuni 35-37 rwhp

Stock 850 with twin Amala 45-48 rwhp.

That's too much performance to throw away, even in my old age!

Same dyno and same dyno operator , Comnoz
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I remember reading this
It's quite a lot of horses to give away
But again,it all depends how the bike is used
 
Only up to page 4. Is it over?

I've never ridden my Norton at 10,000 ft, but I know the jetting that comes from Sudco in the 35mm FCR Norton kits (same thing Matt sells) works as good at sea level as it does at 5400 ft. My guess is that at 10,000 ft I might have to downshift occasionally.

How well does a set of 32mm Amals tuned at sea level work at 10,000 ft? I'm guessing the answer will be absolutely perfect. lol

Some of you guys must have grown up tuning motorcycles without a dyno. It was painfully obvious that the HP was down riding the single carbureted Commando 750 I had. My P11 would run circles around it, which is why I sold the Commando. I didn't have the patience to screw around with the Commando at the time. Too busy chasing the legal tender in Silicon Valley, and had my eyes on modern motorcycles.
 
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Chris Zet said:
.....After suggestion from my carburetor dealer in my hometown I bought 2 Mikuni RS34…flatsliders with accelerator pumps……both slides are operated with one shaft....

Regards, chris
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Mikuni RS34…flatsliders look good but the throttle shaft is toward the rear and because of that I don't think they will fit on a Commando (frame interference).
 
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Cycle magazine March '75 did the single Mik conversion and did back to back dragstrip tests . 13.65 et vs 13.4 and a 4 mph faster terminal speed with the Amals . Test is in the Brookland Books "ultimate portfolio ". They also said , however , that the single Mik was quicker 50 to 80 in top which seems odd given the bhp disparity from Comnoz dyno ?
I've purchased two nice 850s in the past 4 years , both came with single Mikunis, both MUCH more suited MY style of riding with a reversion to twin Amals and acceleration which would see off my brother's single carb 650 Triumph above 75 where it wouldn't before.
Anyhow as has been established , there are myriad opinions and priorities on all this . I love the anodised slides in my ham canned Amals but would love to stick a pair of dellortos on her for real Ron Wood cool! I love the Mikuni VMs on my modified bonnie but won't touch the (brass sleeved ) Amals on the stock looking '68 . I'd like the performance gain of the flat slide FCR Keihins but think they are as ugly as sin , but that's just me .
P.s. The Laverda forum has exactly the same debate around dellortos vs Japanese flatslides, and the PHF Dell is way superior in most respects to the Amals , we (the classic bike fraternity ) are just inveterate tinkerers !
 
Some more Struth

Forgot to mention that the Commando I felt was down on power with a single carburetor on it had a single Amal on it. I can't find any pictures of that bike but want to say it was a monobloc Amal on a Norton intake. The previous owner may have installed the old setup in place of the dual Amals. It was smooth running, but definitely under carbureted, so not a fair comparison to what the people with a larger Mikuni TM-40 pumper, or Dellorto pumper, or whatever. Anyway, it left an indelible impression in my mind that single carburetion on a twin was not for me, so I never pursued it. I have a once bitten twice shy limitation.

I would imagine the current crop of single carburetor setups mentioned in this thread work well with a larger carburetor with jetting that is known to work for the last 30 years or however long MAP and or Sudco has been making single intake manifolds for the old Norton twins.

Where's that 5th page? ;)
 
I have had enough of mucking around with different carbs, now have gone back to my old Amals, full rebuild new jets, needles and pilot jet unclogged the last 12 years been running so called better flatside carbs that was set up for my Norton engine but was a bitch to start cold sometimes, ran well but left carb always had a sticking float problem even after been told how to fix it, it never did fix it, didn't stick all the time but more than it should have.
To me Amal's have always ran well on my Norton with the right jets for my hot motor, turn fuel on, tickle, kick, first kick every time if tune right and the tuning doesn't have to be perfect, the Amal's are easy to work on when needed, are reliable, simple in design, for just being a road rider a new set of Amal Premiers can't go wrong and a lot better for clearing blocked pilot jets.
My original Amal's the slide wore out after a few years of everyday riding from new, ordered a new set of Amal's back in the 80s, they also where a everyday ride until about 12 years ago when I started to think something might be better, I can't afford expensive carbs like the FCRs so budget restrains were a major problem, so back to the replacement old Amal's and after all the years and miles clocked on them the slides are still not showing no wear, after I put them back on even with static tune it fired up first kick then started to run cold on the left cylinder another pull down and soaking in vinegar for a day or two found the pilot jet blocked so a pock out with fine wire and a blow out with compress air the blockage shot out and stung my finger that was over the pilot jet hole, carbs back on bike to fire up first kick again and quick tune by ear.
Just need to take it for a road test as still doing a few more things before that happens the Norton is still on the lift table and need more fuel for it as only enough fuel to get fuel out of the ticklers to start it.
One thing I have found with road going and regular road miles on a Norton with Amal carbs is very important to renew the jets and needles every 3 years running as they wear from vibrations over time, a cheap fix to keep Amal's running great.
If you want simple carbs, easy to maintain and easy to work on, very easy to tune and have your Norton run great if jetted right for your motor then a set of new Amal Premiers jetted right you can't go wrong and with my Norton a big spark from the Joe Hunt maggie work so well with the Amal's.
Plus the Amal's just look right on our Norton's, fit perfect, don't know why I changed as my Norton ran so well with the Amal's, but as they say you learn from your mistakes.

Ashley
 
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