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

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Hi all,
This is an interesting topic, apart from the practicality of the question, it naturally leads to the more complex issue of why we own and ride our bikes at all. Obviously almost every aspect of our bikes, particularly the carburettors are now obsolete and they do nothing that can’t fundamentally be done better by something newer. Yet here we are getting great enjoyment from our Nortons.
There are those amongst us who insist on absolute concourse originality, and good luck to them.
There are some, particularly those involved in various motor sports, who wildly modify their bikes and achieve stunning (if short lived) performance from our antiques.
And then there is the majority of us owners who wish to retain the appearance and heart and spirit of our bikes as they were sold and ridden half a century ago. And this is where I sit on the spectrum. I’m totally happy to make minor modifications that improve our bikes rather dubious reliability, safety and hopefully even lift their performance. The obvious mods that spring to mind are electronic ignition, master cylinder diameter, headlight bulbs and a some internal parts either better designed or manufactured than the bike was originally equipped with. So modified, for all intents and purposes our bikes largely represent what our bikes were like in their heyday.
So where do carburettors fit into all of this? They really seem to occupy a bit of a grey area, some would argue an unacceptable shift from originality, others a relatively minor upgrade. For me carbs are not only a vital major engine component but also strongly affect the visual appearance of the bike and it’s riding characteristics.
Few would disagree that the alternative carbs are superior to our Amals in almost all respects. The question is whether or not the gains in reliability and possibly performance are worth changing the appearance and character of the bike.
This is my opinion
Although primitive, rather poorly built and prone to wear, a well set up pair of Amals is capable of giving a very good performance. They will use more fuel and need a little fettling. I believe that modern Premiers largely get around much of these issues. I think Amals were way past their ‘use by date’ by the time our bikes went out of production. By 1975 the idea of flooding a carb (other than the one fitted to your Victor lawnmowe) was positively archaic but nevertheless that’s how they came.
No matter which way I think about it, fitting a single carb is a retrograde step. Dual carbs not only enhanced performance but we’re an iconic symbol that the bike was designed as a performance machine. Say the word ‘Triumph and the word Bonneville instantly springs to mind. Why, it’s twin carburettors proclaimed performance. As for balancing and tuning, I’m sure an E Type Jag would be easier to live with if we replaced the triple SUs with a single down-draft Stromberg but I’m think we would all agree that that would be missing the point
For me, I would definitely invest in a new pair of Premiums. On the plus side, if correctly tuned they should provide excellent starting (complete with the whiff of petrol and a fuel stain down the side of the bowl). Performance reasonably as good as any twin carb set up and better than a single and maintain the general feel of the bike and look correct.
Before committing to fit a Japanese carb I would ask whether the owner has done everything to ensure the original carbs are in the best possible condition and are correctly tuned. Otherwise they may well be living in a ‘fool’s paradise‘ of comparing badly tuned and worn out Amals with new and correctly tuned Mikunis believing the difference lies solely in the change of brand. Any of us who can afford a Commando as a toy can probably afford the purchase of new Amals if required, tuning and their replacement in thirty years or so, if still around to need it.
Philosophically our bikes eventually become our very own ‘Ship of Theseus’. What constitutes an original bike or eventually what constitutes a Norton at all. Like the ‘farmer’s 100 year old axe’ that has had the head and the handle changed several times, ultimately we could replace our frame with something superior then upgrade to a better engine to take advantage of our new found handling improvements and revel in our ‘modern’ superbike performance but is it a Norton?
Just a thought
regards
Alan
 
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My purpose of having a Norton -> Toy to tune

Das Norton motor has a soft ramp PW3 cam (JS2SS), BSA radiused lifters, high compression short skirt +.040 pistons, long Carrillo rods, lightened crank, ported shaved head, one off 2 into 1 exhaust. I built it, and I have a feeling it's not a slouch motor. The FCRs made perfect sense in my case. They also fit in my frame with a little room to spare. No modification necessary other than a little more padding under the fuel tank. I only responded on this topic because I happen to have experience with the three brands mentioned on a Norton engine or motor whatever one prefers to call the lump. I also have experience with the CRS carburetors, but on an '86 GSXR1100 many moons ago. That was no slouch either.

I did ask Matt for a few FCR dimensions, and he was gracious enough to provide them. I also asked Jim Schmidt for the dimensions of the CRS carburetors, which he too provided. Good people. The FCRs were a better fit dimension wise. I'm not pushing the FCRs at all. I'm only telling a true story. Use what you gotta use. Ride it like you stole it.
 
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.
Matt will have the carbs FCRs (if you go that way) set at a very good starting point....I emailed my engine specifications to Matt and that id been running tuned 34mm mk2 amals on the bike for years which the engine is pretty much identical spec to Gortnippers.....So of you want the next best thing to fuel injection the FCRs are brilliant.....
A jammed throttle slide while riding like you stole it ended my days using that particular carburettor...once you pick all the jap shit stickers off them they also look a bit better...
 
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. ;-)
I currently have dual Amal Premiers, which led me to start the thread last week entitled "My carb fell off. Oof." It was a classic Norton day. Started off fantastic, I'm riding around enjoying the day with a big grin. I think to myself, "Man, it feels SO good to have this bike dialed in and performing," and then I lost power and did a u turn, funny enough, at a fire station at which point the Amal float bowl falls off and dumps gas on my leg. At least if the bike is going to set me on fire it has the decency to do it in front of professionals who can help me.

I have dealt with carbs for decades and almost always eventually succumb to bringing in an expert. I own a carb sync and all the stuff to do it, but I just have trouble getting them to peak performance.

I decided today to go with the FCRs for a few reasons. Mostly, I am hopeful that it will give a subtle boost in performance and is self synching. I struggled with the look of the Japanese sport carb on the bike but to be honest I think it looks pretty cool. On the whole, my bike has lots of invisible upgrades and looks almost bone stock. I like it that way. The Keihins will be the only visible sign of "tampering" and I think they look pretty good. Alas, I'll have to put my ham can in storage.

As for my engine spec.. the engine is about to be rebuilt by Matt at cNw along with the gear box, air intake, electric, brakes and more. I can't say what the spec will be until he gets into it and to be honest I trust him completely to do it right so I'm not really giving him requests or anything. The carb was one of the few decisions I needed to make. Always a class act, he didn't want to oversell me on the Keihins so we discussed what would work best. After what I heard of here and talking with him the Keihins are a good match for me.

Also, after the Keihins are fitted I can take my time restoring my Amal Premiers so they seal properly and work well for someone else. I can keep them around, or maybe I'll sell those to help fund the rebuild :).

I love the conversation here, well put by @Alan L - after how many modifications is it no longer a Norton? I am definitely in that middle camp but my line of thinking is to keep it looking original on the outside and update the things that don't mess with the spirit of the bike. For example, modernizing the electrical components, getting better braking power, and making the motor more of what it already was. Perhaps stopping better makes the bike feel different, and the new power of a rebuilt motor and carb is more powerful than the original, but IMHO these just amplify the spirit of the bike and make it more enjoyable for me.

Last comment on the Amals.. I fully believe everyone that says that when properly tuned they provide great power and the FCRs arent really necessary. While there are countless charming idiosyncrasies of the Norton, I don't find the fickle carb that needs to be flooded one of them. It has stolen too many good riding hours from me.

I probably love wrenching on my bike just as much as riding it, but I like to feel a sense of accomplishment after wrenching. The carb fettling feels more Sisyphean than productive to me.
 
Matt will have the carbs FCRs (if you go that way) set at a very good starting point....I emailed my engine specifications to Matt and that id been running tuned 34mm mk2 amals on the bike for years which the engine is pretty much identical spec to Gortnippers.....So of you want the next best thing to fuel injection the FCRs are brilliant.....
A jammed throttle slide while riding like you stole it ended my days using that particular carburettor...once you pick all the jap shit stickers off them they also look a bit better...

Oh, I forgot to mention that I live at 5,400 feet elevation and the cNw shop is at about 6,900 feet so I have to believe that I have better odds of getting the fuel mix for elevation dialed in better with Matt working on them at altitude. I do almost all my riding at 5,000 - 10,000 ft.
 
Matt will have the carbs FCRs (if you go that way) set at a very good starting point....I emailed my engine specifications to Matt and that id been running tuned 34mm mk2 amals on the bike for years which the engine is pretty much identical spec to Gortnippers.....So of you want the next best thing to fuel injection the FCRs are brilliant.....
A jammed throttle slide while riding like you stole it ended my days using that particular carburettor...once you pick all the jap shit stickers off them they also look a bit better...
Out of curiosity, how different was your FCR setup to stock?
 
Out of curiosity, how different was your FCR setup to stock?
Thankfully they basically came as plug and play...idles nice no hesitation.. plugs checks are good and once I get a chance to get in and have it dyno tuned to see if I can squeeze any more out of them ...will let you know...
 

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Oh, I forgot to mention that I live at 5,400 feet elevation and the cNw shop is at about 6,900 feet so I have to believe that I have better odds of getting the fuel mix for elevation dialed in better with Matt working on them at altitude. I do almost all my riding at 5,000 - 10,000 ft.
Matt is a good bugger
 
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. ;-)
Stage 2 head :oops:

It’s not an old man’s touring bike :rolleyes:

Stage 3 if you don’t mind :cool:
 
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Stage 2 head :oops:

It’s not an old man’s touring bike :rolleyes:

Stage 3 if you don’t mind :cool:
I missed out buying this older magazine recently on trade me that was up for auction ...luckily I found a electronic version
 

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Nortorious, I’m really looking forward to seeing pictures and MUCH more info about your Commando build. It sounds great, and sacrificing an easily replaceable modern bike to do the job right is definitely the right approach (in fact I’ve just done similar, but even more drastic) !

Carry on…
 
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.
I have an 850 with a single mk2 Amal 34. It romps away above 5,000rpm. That's how I first identified a slipping clutch (now fixed), fine until 5,500 revs then the extra torque caused slip. At 5k I get a really nice surge. Don't know above 6k, as that's over 80 in 2nd on my gearing.
 
I have an 850 with a single mk2 Amal 34. It romps away above 5,000rpm. That's how I first identified a slipping clutch (now fixed), fine until 5,500 revs then the extra torque caused slip. At 5k I get a really nice surge. Don't know above 6k, as that's over 80 in 2nd on my gearing.
Have you compared it back to back with well set up twin carbs in good condition ?
 
Have you compared it back to back with well set up twin carbs in good condition ?
I'm not claiming it's faster or better, just in my experience, it's not correct that a single carb is unable to pull well at over 5k. I did test ride one 750 2xAmals, before I bought my bike. It was a few weeks before and TBH, if I do recall anything, the bike I have has more punch at low revs and was less vibration (which could be due to many things).
 
I'm not claiming it's faster or better, just in my experience, it's not correct that a single carb is unable to pull well at over 5k. I did test ride one 750 2xAmals, before I bought my bike. It was a few weeks before and TBH, if I do recall anything, the bike I have has more punch at low revs and was less vibration (which could be due to many things).

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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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.
 
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.
I'd like to add to your list of major "look" of a commando is the polished clock housings and the Z plates
 
A standard Commando crank is probably balanced to suit a rev limit of 4000 RPM. A lot depends on how you intend to use the bike. The taper on the carburetor needle is designed to compensate for loss of vacuum. It also determines how fast the throttle can be opened without the engine gasping. If your usable rev range is between 5000 and 7000 RPM and you have a close gearbox, a lean needle can be very good. But normally road bikes are not set up like that. On a race track, even the slowest corners are usually faster than the speed limit on public roads.
 
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