Cycle Magazine's Test of Single Mikuni v Twin Amals

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1up3down

1up3down

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Regarding the performance differences between a single 34 Mikuni and twin Amals - I have a book with all of the Commando road tests.

In the book is a write up by I believe Cycle or Cycle world magazine detailing the installation of a Mikuni 34 and doing road acceleration comparisons between that and good pair of "well set up" Amals. The conclusion was that the Mikuni set up was faster than the twin Amals from 50-80 mph in top gear roll ons, and that after that the Amals were stronger. Also, quarter mile testing was done and the single Mikuni bike was very close to a half of a second to 3/4 of a second slower than the twin Amals. The article stated that new Amals correctly set up were good for about 4000 miles whereupon the slides would wear away enough of the body bores to promote uneven idling. In addition, the Mikuni bike did not need gas leaking/tickling and had a perfect idle without being subject to mileage deterioration as the Amals were.
IF anyone reading this can direct me to any other real world comparison road testing I would like to read it as this Cycle Magazine article is the only one I am aware of that is comprehensive to include acceleration roll ons and quarter mile testing. Personal bias, seat of pants, comments and opinions are only that when compared to actual bike to bike stop watch performance testing as Cycle took the time to present.
 
thanks , interesting read .... personal opinion here , for me I know after almost 50 years on 2 wheels no way could I judge 1/4 mile rip slower by a second or even 2 .... so for me the article means , single VM34 was/is best choice for my rides .... as mentioned earlier I got no cause to try and ride an almost 50 year old bike to it's and my limits I have perfectly good more modern bikes for covering long distances very fast and comfortably .... I do admire those who still cling to that notion of getting all they can from their old bikes , pretty darn cool , just not my thing .... interested to hear from others on their own street performance preferences and mods made to attain ....
Craig
 
1up3down writes: "Personal bias, seat of pants, comments and opinions are only that when compared to actual bike to bike stop watch performance testing as Cycle took the time to present."

I don't know about that. Ever heard the phrase "the plural of anecdote is data"?
Spend 15 minutes in the archives and you will find dozens of credible references to single mikunis providing more torque lower down and less hp higher up. With the additional comments that for the way most of them ride, it doesn't make any practical difference.

What else do you need? Besides, after you crunch your numbers, should you find them, you and you alone must make the subjective determination that it does (or does not) matter for how you ride your bike.

Stephen Hill
 
1up3down said:
Regarding the performance differences between a single 34 Mikuni and twin Amals - I have a book with all of the Commando road tests.

In the book is a write up by I believe Cycle or Cycle world magazine detailing the installation of a Mikuni 34 and doing road acceleration comparisons between that and good pair of "well set up" Amals. snipped
Click to expand...

Could you please enlighten me with the title of said book? Or the year and month of the article in Cycle?
 
Could you please enlighten me with the title of said book? Or the year and month of the article in Cycle?
---------------------------------

Certainly, The name of my book is Norton Commando, Ultimate Portfolio
published by Brooklands Books
On page 144 is an article titled Norton Commando: The Mikuni Conversion
it is a reprint from Cycle magazine, March of 1975

edit: Very interesting book, it has all of the magazine Commando road tests, British and American, from 1968-1976, all Commando models
 
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Top gear roll on at 50mph reminds me of the story of how the US allegedly invented tests that suited Harley’s!

Surely anyone opening the throttle at 50mph in top gear on a Cdo is not interested in maximum acceleration?

As I understand it, a single carb does have a theoretical better gas flow at low rpm as the charge is not doing a constant ‘stop-start’ as it would if it were only feeding one cylinder.

But in the practical world, I’d suggest that the Mikuni can give a better low end torque simply due to giving cleaner carburation vs Amals that may be worn or badly adjusted. I would also suggest that a Mikuni (as with Keihin) is a better made instrument than an Amal. Let’s not forget that the concentric carb was designed to be as cheap as humanly possible, as demanded by the Brit OEMs in the 60s.

It’s perhaps a bit like the electronic ign situation... anyone replacing worn out points will think their new EI is the dogs bollox, irrespective of how good it is against other EIs.

Similarly, anyone replacing worn out and perhaps badly adjusted Amals with a new and well set up Mikuni surely CAN’T FAIL to think it’s fabulous!

So, I think we mix many issues here gents: good carbs vs cheap carbs, new carbs vs worn carbs, well set up carbs vs badly adjusted carbs, etc and we lump them all in with the twin carb vs single debate.

A real twin vs single test would test twin Amals back to back with a single Amal, twin Mikunis back to back with a single Mikuni etc.
 
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We all have our own ideas about carbies, Amals carbies are a simple carb, once set up they work really well and Commandos were designed to run twin carbs, they are a high performance twin cylinder motor for the times, I have owned my Norton for 43 years now, I am no carbie expert but I do know how set up twin Amals I have run them with worn out slides and still have them perform well.
Some say single carbs idle better but so do twin Amal's even with worn out slide, ignition systems play a very important role with carbies they work together and set up right is important to have a good running Norton, some say it so hard to tune twin Amals, if they are jeted right and you do it by the book they are simple but a lot of people don't want to put the time into it, I am not a fan of single carb on a Norton, only if its a single cylinder of course, but a lot of people blame carb problems when its igniton problems or something else and another thing is people don't replace needles and jets they do wear out from vibrations I replace mine every 2 or 3 years depends how many miles I do, hard mount carbies on a Norton will wear out the the jets and not many think about that.
This is my opinion but I have never had much troubles with running twin amal carbs or turning them, they are a simple carbie.

Ashley
 
ashman said:
We all have our own ideas about carbies, Amals carbies are a simple carb, once set up they work really well and Commandos were designed to run twin carbs, they are a high performance twin cylinder motor for the times, I have owned my Norton for 43 years now, I am no carbie expert but I do know how set up twin Amals I have run them with worn out slides and still have them perform well.
Some say single carbs idle better but so do twin Amal's even with worn out slide, ignition systems play a very important role with carbies they work together and set up right is important to have a good running Norton, some say it so hard to tune twin Amals, if they are jeted right and you do it by the book they are simple but a lot of people don't want to put the time into it, I am not a fan of single carb on a Norton, only if its a single cylinder of course, but a lot of people blame carb problems when its igniton problems or something else and another thing is people don't replace needles and jets they do wear out from vibrations I replace mine every 2 or 3 years depends how many miles I do, hard mount carbies on a Norton will wear out the the jets and not many think about that.
This is my opinion but I have never had much troubles with running twin amal carbs or turning them, they are a simple carbie.

Ashley
Click to expand...

I agree!
Dave
 
I was told by my dad that the reason for the 360 degree crank (originally by Edward Turner?) was to allow fitment of a single carburettor (even spaces between inlet pulses) due to fuel shortages in the period around WWII. The Japs had a better idea (much later) with the 180 degree crank but... it just doesn't sound right. The modern 270 degree crank also doesn't - at least to my ear. A mate with a 2006 triumph scrambler still gets edgy when I refer to his bike as a Triumphati. That was, until he shut his bike down at a local music festival (Agnes Water Blues & Roots) only to have a bloke charge into the parking area demanding to know where the 900SS was that he'd heard arrive.
So... if my dad was right, the noise we've all come to love was borne of fuel shortages in the war years and the UK inability to break the paradigm thereafter.
Hope this is of some interest/amusement to some.
Cheers
Rob
 
And would a single 34 Mikuni do a 15mph non snatch crawl in top gear as per the single Amal carb on a Norton 750 Atlas?
 
Bernhard said:
And would a single 34 Mikuni do a 15mph non snatch crawl in top gear as per the single Amal carb on a Norton 750 Atlas?
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I don’t know the answer.

But I have always wondered what the point of these claims, frequently included in old skool road tests, were?

Why would you ever want to do that??
 
robs ss said:
I was told by my dad that the reason for the 360 degree crank (originally by Edward Turner?) was to allow fitment of a single carburettor (even spaces between inlet pulses)
Click to expand...

That, I would agree with, and is why the majority of British vertical 4-stroke twins had 360 degree cranks as it's simple and works reasonably well.
Although a carburettor per cylinder would be considered normal by today's standards it wouldn't have been at that time, even on 4 cylinder engines.
ET was insistent that Triumphs should be tuned for normal road use and if an owner decided he wanted to go racing he could buy the necessary parts from Triumph.
After all, if ET had got his way (which he normally did, but not on this occasion) then the Triumph Bonneville would not have gone into production as he considered it to be too highly tuned.



robs ss said:
due to fuel shortages in the period around WWII.
Click to expand...

I don't exactly agree with that, as the Speed Twin was designed and went into production before WW2.
 
I believe Bert Hopwood was working with/for Edward Turner around the war years and probably influenced Bert's 360 degree crank on the Norton model 7 which eventually, although not necessarily, flowed into the Commando. It's interesting as at least we have a choice of single/twin carbs - other crank configurations don't.
 
As has been said many times, a properly set up pair of Amals works perfectly. The last time I had to adjust my Amals was...well...uh...I can't remember. They just work. ;)

BUT, if you are talking about restoring an original set of Amals, as opposed to buying a new set, you do need to have them sleeved and also true all mating surfaces because odds are that they are warped from previous over tightening and won't seal as they should. For that matter, the same is true of the intake manifold - both ends should be trued. You can do that on a sheet of glass with Wet/dry sandpaper, ending with 400 grit. Some folks go to 600 which is fine but not really necessary in this application.
 
If fitting a single carburettor instead of duals gave you stronger mid-range power, wouldn't you raise the overall gearing slightly ? That way you take advantage of the better mid-range. Until I owned my 850 motor, I always believed that lowering the gearing gave better acceleration. I was surprised when I raised the overall gearing and it accelerated faster. - Works better with a close ratio box.
 
I would never have rigidly mounted carbs on a race bike - too prone to flooding due to vibration - makes the motor sluggish.
 
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