single carby amal

Fast Eddie said:

It is the manifold that is the bottleneck to flow when fitting a single carb on a Norton.

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With a 2 into 1 exhaust system on a Commando, the tail pipe resonates at twice the frequency of one of the header pipes. So the length of the tail pipe needs to be a multiple of the length of the header pipe. On most bikes which have 2 into 1 exhausts that multiple is 1 - the tail pipe is the same length as the header pipe. Similar probably applies to single carb on a twin cylinder motorcycle. With twin carbs - at TDC when both valves are open, there is flow across the top of the piston and into the exhaust, the pulse from Kadency effect pushes mixture back into the cylinder from the exhaust. Waves don't like going around corners. But a single carb is much easier and cheaper to tune well.

I like Mk2 Amals because I think they are made from better material. However I do not use Amal needles or needle jets. In the 1970s, every new model of Japanese motorcycle had different needles and needle jets. The Mikuni range of jets and needles is very good, and some fit MK2 Amal carbs. A 32mm MK2 Amal carb is usually very similar to 32mm Mikuni carb .
I don't know how anyone makes sense out of Amal needles.

I bought my 34mm Amal carbs and manifolds from Mick Hemmings many years ago. and requested methanol kitted ones. The needles and needles jets were absurd, and the mounting rubbers cracked. I replaced the rubbers with the correct ones, and used 6D Mikuni needles and made my own needle jets out of brass hex, using a mix of metric and number drills. It is not possible to buy needle jets in small enough size steps, even when using methanol which gives twice the room for errors. The leaner needles actually make a difference.
When I raced fairly regularly during the 1970s, I thought I knew what I was doing, but actually I really had no idea.

acotrel said:

I like Mk2 Amals because I think they are made from better material. However I do not use Amal needles or needle jets. In the 1970s, every new model of Japanese motorcycle had different needles and needle jets. The Mikuni range of jets and needles is very good, and some fit MK2 Amal carbs. A 32mm MK2 Amal carb is usually very similar to 32mm Mikuni carb .
I don't know how anyone makes sense out of Amal needles.

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I wonder why a mikuni VM is similar to a mk 2 Amal

jay*mac said:

has anyone tried the single amal premier carb set up coming out of the US
FAIR SPARES NEW ENGLAND
if so what is your thoughts
i would like to give it a go on my mk111 850 which is in a featherbed bolt up 52 frame
i have 2 worn out mk1 concentrics at the moment
thanks jason

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Hi Jason and all,
I am absolutely confused by the number of different carb set ups people put on our bikes.
Most of us now own our bikes as they invoke a degree of nostalgia and it is rewarding to own a fine and still fast bike that can bridge the divide between the modern riding experience and the feel of yesterday.
Of course there are many little modern tweaks we can give our bikes to improve their reliability and their performance, things such as electronic ignition, better lights or perhaps even a starter motor if our ageing knees fail us.
But I wonder about carburettors. I am no expert but it seems to me that the original twin Amals can be set up to provide the bike with easy starting (mine is absolutely first kick), good idling (I fall down here) and strong performance through the range. In other words everything you really need.
I know that the Amal is primitive and crudely built device that lacks sophistication and wears out surprisingly quickly. I would suggest that management should have had their arses kicked for battling on for as long as did with the them and if Amal couldn’t or wouldn’t pull their socks up they should have found an alternative supplier. As an aside, I have no idea whether the Mk 2 on later Bonnie’s were a substantial improvement. Nevertheless , these were the carbs our bikes came with. Having to tickle a carburettor on anything better than a lawn mower in the mid seventies, you just have to be kidding but it is all part of the dubious charm of our bikes.
A harsh critic of Pommie bikes could point out that those criticisms apply to much more than our carbs. So, my question is, why spend considerable amounts of money to convert your bike to a non standard set up when for the same money the bike could be equipped with 2 brand new Premier carburettors that would probably see most of us out, considering our average age and miles covered.
As for performance, I might be aging and perhaps not quite so stupid as I was in my youth but I definitely want to know the bike I’m riding is capable of the best performance that it is reasonably capable of. I should add that unfortunately I will never be able to recreate the thrill of my youth as the many extra kilos I now carry around my girth must make my acceleration substantially worse than it was when I was young and my bike was new. In other words, I want all the performance I can get.
When our bikes were made twin carbs was a pronouncement to the world that you were astride a high performance machine. It was integral to the spirit of our bikes. Now, there will be those who know far more than me who will state that a single carb machine can perform just as well but that is missing the point. There will be those who say how much easier it is to tune single carbs especially if they are superior Japanese units but again that misses the point. If you are capable of keeping a 50 year old Norton in fine fettle, you are certainly capable of tuning (or having them tuned) a couple of old Amals.
Just a thought
Alan

My 74 850 came to me with single mk2 amal. Ran the bike with it for a while. Got it running and starting ok. Maybe on leaner than it should have been. Had a scare one day after spirited riding, stopped for fuel up, went to give throttle a twist prior to kicking over from force of habit, grip did not return to closed position, stuck near wide open scratched head before discovering fuel bowl was too hot to touch. A few minutes later throttle just snapped shut on its own as it cooled down.
After that, I fitted the thick type insulators btwn head and mani ports and another btwn carb and mani to help keep heat soak down. Never happened again. But went to dual mk1 preimers next season and happy with them.

On Jim's dyno , iirc the single Mikuni setup was about -8 or so rwhp
That's a lot on a 45 rwhp machine.
My bike has always run great on twin amals and I don't seem to need to fiddle with them, so I have a tough time seeing a good reason to go to -8 rwhp.


Glen

And there was me thinking the single/twin carb debate had been flogged to death
Could be worse, could be what's the best oil for an SU dashpot
Or best oil for carburettor cable/cables !

worntorn said:

On Jim's dyno , iirc the single Mikuni setup was about -8 or so rwhp
That's a lot on a 45 rwhp machine.
My bike has always run great on twin amals and I don't seem to need to fiddle with them, so I have a tough time seeing a good reason to go to -8 rwhp.


Glen

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But what was the difference at 2800 to 4500 rpm, which is where I spend a lot of time?

Mine came to me with a single 34mk2 Amal. I thought at the time I got it, I would go to 2 carbs. But, it runs great, so I've just not seen the point. I really wouldn't have hesitated if I thought the motor was showing signs of struggling.

I have a CB750K6 with 4 Keihins. My Commando is quicker, due to its low down torque.

I don't know exactly where the loss was. I would guess that it starts at the bottom of the midrange and increases with rpm.
Like most riders, when I really need power to pass a string of slow cars, I use full throttle and the top part of the rev range. I imagine that's where the biggest loss is with the single carb.

Mart UK said:

But what was the difference at 2800 to 4500 rpm, which is where I spend a lot of time?

Mine came to me with a single 34mk2 Amal. I thought at the time I got it, I would go to 2 carbs. But, it runs great, so I've just not seen the point. I really wouldn't have hesitated if I thought the motor was showing signs of struggling.

I have a CB750K6 with 4 Keihins. My Commando is quicker, due to its low down torque.

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Single vs twin carbs should not be a debate. It is not a ‘right or wrong’ answer. It’s about understanding the differences and then each person deciding what works best for them.

Mine had a single 36mm Mik when I bought it, it noticeably fell away above 4,500 and badly so at 5,000. This was on an otherwise good condition and stock 850.

A single carb is cheaper and easier to dial in. And a well dialled in single carb will certainly run better in the 2,800-4,500 range than poorly set up and perhaps worn out twin carbs.

But… we have to acknowledge / remember that most people who fit a nice new Mikuni are replacing a pair of 50 year old, partially blocked and probably shagged out Amals. So they are OBVIOUSLY gonna be impressed with the new carb !

I’d put money on it that a single carb does NOT perform any better in the lower rpms on a Norton than correctly set up twin carbs in good condition. I have never tested back to back, but that’s what my money would be on.

The other thing I often point out is that ‘higher rpm performance’ does NOT just mean top speed! In reality very few of us are interested in outright top speed on road going Commandos (I’m certainly not). But… acceleration is maximised using higher rpms in all gears. For me personally, that’s where the fun is, and it is something I’d never voluntarily sacrifice. But if someone genuinely never revs above 5,000 in any gear, then a single carb (of any type) will be perfectly fine for them.

The last thing I’ll say here before anyone gets the wrong idea… is that despite the above point of view… I am a Mikuni fan. I think they are very nicely made instruments, offer remarkable value for money and are thus superior to Amals, and I run a pair on my 1330cc Egli Vin very happily indeed.

I wouldn't like to restrict my well tuned Norton, love it when I open the throttle and instant get up and go, no need to open the throttle fully but its good to know its there if I need it, I can't understand why so many have problems setting up twin Amals, I am not interested in top end flat out speed but I love giving it to it in my 2 favorite gears, second and third, I call them my play gears, I have played with other dual carbs on my hot motor but found out well set up Amals jetted to my motor and open exhaust works the best with that big spark from the JH magneto, starts first kick every time, tickle, kick, go and it never hesitate when I do open that throttle and being a light weight and shorter than a Commando frame the hardest part is keep the front wheel down.
Well I am mature now and don't do silly things like I use to in my younger days, well not as much lol, I can ride around the suburbs quite good in top gear at 40 MPH and open the throttle and it just goes, even quicker if I drop it back to 3rd, but a well tuned 850 is a fun bike, I have so much fun on my hot 850 light weight Featherbed, puts a smile on my dial every time I take it out, its not built to be pussy footed around, but its good in the suburbs, up in the tight twisties and on the open road, it does everything I ask of it and more, so I couldn't restrict it to one single carb no matter what.

Ashley

My Austin mini with 1275 cc powerplant only has a single SU carbie and does quite well...and it's got a funky inlet manifold and head flow is not cross flow, so gases must change direction 180 degrees going in and then out.

One other thing I recall on single mk1 amal, seemed to get a higher fuel mileage per gallon.

Tornado said:

One other thing I recall on single mk1 amal, seemed to get a higher fuel mileage per gallon.

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I have always had great mileage with twin Mk1 Amals on my 850. Your final drive sprocket makes a big difference. I use a 21 tooth as opposed to the 20 tooth, that most Canadian/US, 850's were setup when sold. My 750 came with a 19 tooth sprocket. The fun factor was great but your revs at 70-80 mph are higher, therefore your looking for a gas station a lot sooner.
For my touring Interstate that is what I have for my setup. I us a Maney belt drive as well.

What are you guys using for 2 in to 1 Fuel lines and where is the best place to get them?

speirmoor said:

What are you guys using for 2 in to 1 Fuel lines and where is the best place to get them?

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I just ran a line from each petcock to a double spigot banjo on carb.

CanukNortonNut said:

I have always had great mileage with twin Mk1 Amals on my 850. Your final drive sprocket makes a big difference. I use a 21 tooth as opposed to the 20 tooth, that most Canadian/US, 850's were setup when sold. My 750 came with a 19 tooth sprocket. The fun factor was great but your revs at 70-80 mph are higher, therefore your looking for a gas station a lot sooner.
For my touring Interstate that is what I have for my setup. I us a Maney belt drive as well.

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Recall seeing high 50's mpg (US gal) with single, more like lower 50's with dual setup.

My favourite show on TV is 'The Antiques Road Show'. Some people say it is the most boring. But I love to see the things the Poms have got. I really like their mindsets. A Norton Commando is not a Japanese motorcycle. If it was, it would have had 6 gears , fuel injection and a programmable ignition system in the 1970s. However it is still 'fit for purpose'.
I am not bragging, but when I built my Seeley 850, I never expected it to do anything. However it has responded to tuning very well, and I now actually believe in it. - What it can do in a race is just a bloody joke, for something which is so horrible. It took me a longer time to learn how to use it properly - it is different.
Normally we do not flick a bike into a corner while braking, then immediately accelerate using full throttle, all the way around it and up the next straight. - It is absurd,