Single Mikuni or Amal Mk II

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Stephen Hill said:
flatfan, who started this thread, wrote:
"Unfortunatly the bike didn´t have the original Amals when I bought it back after 30+ years, it was equipped with two Mikunis which are from a snowmobile engine I´ve been told. Probably made for 2-strokes. They probably work but I´m going for a single."

Sounds like he has never run the bike. Twin Mikunis are a valid carb option for a Norton, once they are jetted correctly.
https://www.mikunioz.com/shop/norton-750-850-commando-dual-vm34-carb-conversion-kit/?v=3e8d115eb4b3

I would run the bike and see how the twin Mikunis work before deciding to swap them out.

Stephen Hill
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Stephen, I owned the bike from 1974 to 1984, then sold it with the original Concentrics. Bought it back 2 years ago and then these Micunis were on it, they´re not VM:s but some other model, as I wrote I believe they´re made for 2-strokes. I did drive the bike with those carbs just a short way before starting to take it apart and it worked, but I didn´t drive it enough to make any in depth analysis.
 
Fullauto said:
You see posts on here about people trying various carbs, both dual and single, and then throwing them away after a short while with nothing but problems and then declaring them junk. They would never use them again. The problem usually is that they are not jetted correctly. I know that with my VM 34 Mikuni, it was on the bike when I got it and I have done some 60,000 miles since, that it has been virtually trouble free in that time. I don't know about Anals,but my 850 will run very well over a broad range of jetting. I thought that I knew a little bit about jetting, but until I fitted an O2 sensor and gauge, I realised that I knew virtually NOTHING about the subject. Where I thought I was going the right way with jetting, the O2 sensor showed me that it was the WRONG way. Even a run on a dyno with a sniffer up its bum, showed that these things are not perfect either and showed that at lower throttle openings, the bike ran very lean. When I fitted the sensor, in the correct place, it showed exactly the opposite. I fitted the sensor to a spare set of pipes and planned to take it off when I found out what I wanted to know. However, it is such a useful device that I now plan on keeping it fitted.
People say "the bike was running rich, so I fitted a smaller main jet". This is generally wrong. To jet your bike correctly, you need to start with idle settings and work your way up, with main jet choice and air jet choice last. The air jet is not part of the low speed circuit, but a fine tuning method for the main jet. This is because the lower speed settings add fuel at ALL throttle openings, whereas main jet choice does not affect any of the low speed circuits. With my current settings, my 850 is an extremely pleasant, trouble free runner, with very good economy. 65mpg (Imperial). And, as people are aware, I am not interested in what happens in the top operating range of a Norton "powerband".

So, whatever carb setup, please take the time to jet correctly and I'm sure that whatever you choose, it will do the job well. Unless it's an Anal.

Oh, one more thing. The Mikunis generally come jetted very rich for the application, obviously so you don't seize your bike, so there is a lot to be said for correct jetting.
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This is really good info, very interesting!
 
Fast Eddie said:
So, back to Mik vs MK2 Amal...

MK2 Amals can be made to work very well. There are thousands of late Triumphs our there, twin and single carbs, running perfectly on MK2 Amals.

But... the mistake people make (me included) is thinking they’re just MK1s with a different cold start system. They’re NOT. Read up stuff on t’internet by John Healey. He’s a Triumph man, but gives great insight into the MK2 carb.

The other difficulty is having good data for baseline for settings, especially on a non standard application like yours. It’s bad enough on standard applications...

Read the thread I posted in the Triumph pages on this forum. I just went through a real f*cking headache simply trying to get my new, stock MK2 carbs to work on a rebuilt, stock T140. I got there eventually after doing a lot of research, spending a lot of time on trial and error, and spending a lot more £ on new needles, jets and chrome brass slides. This was despite Amal selling the carbs as being jetted for a T140 in the first place!

The single Mik has the advantage of being sold by trusted Norton suppliers who should therefore provide a good baseline setting. Indeed, hopefully plug n play.

And, having worked with both carbs, IMHO the Mikuni is a better made, higher quality instrument.

So, on balance, if I was fitting a single carb (which I would never do again as it killed performance dead at 5,000rpm. It’s NOT only about performance above 80mph... it seriously reduces how quickly you can accelerate to 80mph, which for me, is a huge part of the fun of a good Cdo) I would fit the Mikuni.

All only IMHO of course.
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I will look up that info Eddie, thanks!
 
Fullauto said:
You see posts on here about people trying various carbs, both dual and single, and then throwing them away after a short while with nothing but problems and then declaring them junk. They would never use them again. The problem usually is that they are not jetted correctly. I know that with my VM 34 Mikuni, it was on the bike when I got it and I have done some 60,000 miles since, that it has been virtually trouble free in that time. I don't know about Anals,but my 850 will run very well over a broad range of jetting. I thought that I knew a little bit about jetting, but until I fitted an O2 sensor and gauge, I realised that I knew virtually NOTHING about the subject. Where I thought I was going the right way with jetting, the O2 sensor showed me that it was the WRONG way. Even a run on a dyno with a sniffer up its bum, showed that these things are not perfect either and showed that at lower throttle openings, the bike ran very lean. When I fitted the sensor, in the correct place, it showed exactly the opposite. I fitted the sensor to a spare set of pipes and planned to take it off when I found out what I wanted to know. However, it is such a useful device that I now plan on keeping it fitted.
People say "the bike was running rich, so I fitted a smaller main jet". This is generally wrong. To jet your bike correctly, you need to start with idle settings and work your way up, with main jet choice and air jet choice last. The air jet is not part of the low speed circuit, but a fine tuning method for the main jet. This is because the lower speed settings add fuel at ALL throttle openings, whereas main jet choice does not affect any of the low speed circuits. With my current settings, my 850 is an extremely pleasant, trouble free runner, with very good economy. 65mpg (Imperial). And, as people are aware, I am not interested in what happens in the top operating range of a Norton "powerband".

So, whatever carb setup, please take the time to jet correctly and I'm sure that whatever you choose, it will do the job well. Unless it's an Anal.

Oh, one more thing. The Mikunis generally come jetted very rich for the application, obviously so you don't seize your bike, so there is a lot to be said for correct jetting.
Click to expand...
So, who makes your O2 sensor? I would like to give it a try
 
I don't remember. But I don't think it's important. There's plenty of sensors and gauges out there. I fitted the bung on my right hand header pipe facing straight up between the gearbox and the engine where it is reasonably innocuous. If you want any more info. just PM me or, preferably post something so we can share the knowledge.
 
seattle##gs said:
A friend bought a pair of premiers and we tried to install them a few days ago. Found out they would not tickle. The roll pin was too short. My solution was to take a piece of welding rod, turned it down and tapered it, and tapped it down the center of the roll pin and cut it to length. Now the carbs will flood in 3 seconds. But it should have been right in the first place.
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The roll pin wouldn’t have been too short. It’s been pushed too far up into the button. It’s quite easy to pull it out a little bit to fix the problem.
 
Fullauto said:
I don't remember. But I don't think it's important. There's plenty of sensors and gauges out there. I fitted the bung on my right hand header pipe facing straight up between the gearbox and the engine where it is reasonably innocuous. If you want any more info. just PM me or, preferably post something so we can share the knowledge.
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Sounds better than my Koso ,the bung has to welded in approximately 200mm from the head you can't really hide the bung not that it matters on my bike!!
 
Triton Thrasher said:
The roll pin wouldn’t have been too short. It’s been pushed too far up into the button. It’s quite easy to pull it out a little bit to fix the problem.
Click to expand...
Triton Thrasher said:
The roll pin wouldn’t have been too short. It’s been pushed too far up into the button. It’s quite easy to pull it out a little bit to fix the problem.
Click to expand...
It was a strange thing. The aluminim button would not push all the way down and that was both carbs. I tried to pull the pin with out much luck.
 
Fullauto said:
I don't remember. But I don't think it's important. There's plenty of sensors and gauges out there. I fitted the bung on my right hand header pipe facing straight up between the gearbox and the engine where it is reasonably innocuous. If you want any more info. just PM me or, preferably post something so we can share the knowledge.
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I looked them up and there are several to choose from. I would like to know which one is best.
 
A single carb is easier to get jetted right than twin carbs, and Mikunis usually have a better range of needles than Amals. A well tuned single carb might be better than poorly tuned twin carbs. However I would never use a single carb on a Commando, because I am patient enough when I tune a bike, to get it right. I like Mk2 Amal carbs, but I use Mikuni needles.
 
I bought the Amal MK2s 25 years ago and could not get them to work. I went back to the original Amals and lived with the hard starting sputtering and inconsistent idle for another 25 plus years. I just installed the Wassell copies of the Amals and they are excellent. The ones set up for the Mark III came with jetting that worked perfectly with my Mark III. For 300 bucks a pair, they solve a lot of problems.

 
montelatici said:
I bought the Amal MK2s 25 years ago and could not get them to work. I went back to the original Amals and lived with the hard starting sputtering and inconsistent idle for another 25 plus years. I just installed the Wassell copies of the Amals and they are excellent. The ones set up for the Mark III came with jetting that worked perfectly with my Mark III. For 300 bucks a pair, they solve a lot of problems.

Click to expand...


$300/pair? I see the 850-spec here for $245 USD:

https://thebonnevilleshop.com/norto...t-amal-premier-type-pair-pn-tbs-09-0466-0477/
 
Many years ago Les Emery of the Norvil Motorcycle Company claimed it was impossible to get the Concentric Mk2 to work properly on a Commando.
 
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