Regarding Single Carb Setups On Commandos

Craig said:

I think maybe over these last several years with many threads on this topic we can agree that there are prolly 3 or 4 distinct groups with opinions .... what appears to be lacking is the understanding that we all have different wants , likes and wishes ... let’s just acknowledge that some prefer the original twin carb setup while others prefer the simplicity of a single , still another group warms to the flat slide in single and twin setup while at least one individual swears by FI .... it great to trade ideas and theories but in the end like most things , to each his/her own and I say more power to them ..... in my case I swear by the single vm34 Mikuni .... if I need more speed I ride a different bike

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excellent current info tho for the always incoming new folks, just so happens i've been all over carb options info the last few days, most of it all new to me, including the wassell deal,

also looks like there was quite a stir with the pwk/s..

then i saw these..
http://www.keihin-fcr.com/index.php?route=product/category&path=127_145

Just because you opt for the simplicity of a single VM34 there is no reason that you shouldn't strive to get it performing as well as possible. With some work it should be possible to achieve a decent compromise and with more younger owners coming along who haven't got a masters degree in Nortons, let them start on the simple stuff and graduate to the tougher options. For me that was investing in a strobe and carb balancer. 75% of Norton issues solved at a stoke

Yup all the above comments true , I was hoping that maybe some of the carb choice controversy could be avoided that all .... as far as my bike , I get great low and midrange response with my current setup ... it tops out @ exactly 92 mph on my fully functional rebuilt original Speedo , very happy with that ... I use a Uni-pod air filter it is easy to stuff the end of it in beside battery tray ... any faster and I take one of the newer Italians , the Norton is for Sunday morning solo backroad blasts , within it’s limited performance parameters

gripper said:

Just because you opt for the simplicity of a single VM34 there is no reason that you shouldn't strive to get it performing as well as possible. With some work it should be possible to achieve a decent compromise and with more younger owners coming along who haven't got a masters degree in Nortons, let them start on the simple stuff and graduate to the tougher options. For me that was investing in a strobe and carb balancer. 75% of Norton issues solved at a stoke

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that would include lotsa older folks, back in the game often after loooong layoffs

84ok said:

http://www.keihin-fcr.com/index.php?route=product/category&path=127_145

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Some mighty expensive bragging rights those FCRs. I'm sure that they can be justified at some stage of lowering the Commandos reliability, er making this old horse a bit more spry, at super legal speeds.

A few Mikuni 34 notes:

1) If you think your float bowel is drying up at higher RPMs, then purchase a larger float needle/seat, they are available in many bore sizes.

2) The 34 VM Mikuni is NOT a power adder; the conversion is meant for the rider that is trying to gain simplicity and likes to play on roads where speeds in the 40 to 80 MPH range provoke all the adrenaline the rider desires, with amazingly brisk acceleration under and in that range.

3) But, just like anything else, if 34mm is good then bigger is better, why stop at 34? Any single carburetor larger than a 34 will need to be fit in a Commando sideways; someone was talking about notching the backbone to accommodate some iteration of a monster TM; to me that ranks up there with removing your front brake to save weight.

4) Tuning a single Mikuni is a piece of cake, if you vary one parameter at a time. You don't need a chassis dyno with exhaust gas sampler (they can make short work of tuning), but you do need to employ the scientific method of trial and observance.

Could not agree more , thanks ....

I’m not sure what you’re getting at with the FCR and “lowering reliability” jibe above, but it’s pretty baseless in my experience.

Mikunis are good carbs no question, I had one on my Cdo. But there’s no doubt they stifle performance. And it’s not just max speed that’s effected guys, a single 34 or 36 Mik (mine was 36) just can’t feed enough mixture into the engine at higher revs / throttle positions. Which means, it really blunts acceleration. Thats one of my favourite parts of riding the Cdo. So that’s why I didn’t like mine, I rarely go much over the ton these days, but I love getting there ASAP!

As has already been rightly said, carbs are one of those ‘to each his own’ choices, and they’re only bolt ons, and easily reversed / swapped.

Personally, I would advise those in pursuit of ‘simplicity’ to dig a little deeper into their real motives before assuming that ‘simple = single’.

What does simple mean? If it means simple to operate, easy to start and maintenance free, well, you have just described life with a pair of FCRs!

Being linked, they NEVER go out of synch.
The accelerator pump acts as a perfect cold start device.
Even with a hot cam and 10.5:1 compression ratio my bike is a 1-2 kick starter, hot or cold.
Tickover is like a Honda, even after a gruelling track session on the hottest U.K. day in decades.
(Tri Spark also helps with all of the above)
Installation is as easy as ABC.
And if you by a kit from cNw it comes correctly jetted and with a flow bench designed manifold.
Oh yes, and they liberate serious performance potential.

The only downside is they’re not cheap.

Some say the slides rattle. I say I’ve NEVER noticed that over the normal engine and peashooter noise!

Here’s mine when they were first fitted 5 yrs ago:

I guess each to their own on the single verses twin carb argument although the actual question was "how to improve the performance of a 34vm"
But you do get to read funny things like notching the back bone of the frame!!! Nobody has said that to my knowledge?
Notching the gusset plate has been mentioned but not the back bone,
Plus a TM carb in this instance a 40mm being described as a "monster carb" it's only 6mm bigger than the VM 34 that we are talking about and smaller than a pair of 32 concentrics
There is no doubt the VM 34 does make a commando a nice bike to ride I think we all agree on that,
But for me it makes the bike dull ,the tm40 does all that a single vm34 will do plus it will also do what the twin concentrics will do,I enjoy riding and thrashing my commando,I built it from parts if it ever blows up it'll be my fault and I'll build it better next time
I have 2 faster bikes,I have owned much much faster bikes but there is something about a commando ....as a footnote all the things I have stated are things that I have actually done myself they are not theories/assumptions etc

Removing, or severely hacking, the gusset plate was mentioned as a necessity of fitting a longer, straighter, manifold. Cutting into the main frame spine would be something I would not be prepared to consider personally!

But at the end of the day, the only really helpful answer to the question “how do I increase the top end performance of my 34mm Mikuni equipped Cdo” is... replace it with something more suitable for that need!

Be that twin Amals, twin Mikunis, a big flat slide, twin flat slides, etc.

(And remember that ‘top end’ means higher revs / higher throttle openings in all gears, NOT just top speed)

Fast Eddie said:

I’m not sure what you’re getting at with the FCR and “lowering reliability” jibe above, but it’s pretty baseless in my experience.

Mikunis are good carbs no question, I had one on my Cdo. But there’s no doubt they stifle performance. And it’s not just max speed that’s effected guys, a single 34 or 36 Mik (mine was 36) just can’t feed enough mixture into the engine at higher revs / throttle positions. Which means, it really blunts acceleration. Thats one of my favourite parts of riding the Cdo. So that’s why I didn’t like mine, I rarely go much over the ton these days, but I love getting there ASAP!

As has already been rightly said, carbs are one of those ‘to each his own’ choices, and they’re only bolt ons, and easily reversed / swapped.

Personally, I would advise those in pursuit of ‘simplicity’ to dig a little deeper into their real motives before assuming that ‘simple = single’.

What does simple mean? If it means simple to operate, easy to start and maintenance free, well, you have just described life with a pair of FCRs!

Being linked, they NEVER go out of synch.
The accelerator pump acts as a perfect cold start device.
Even with a hot cam and 10.5:1 compression ratio my bike is a 1-2 kick starter, hot or cold.
Tickover is like a Honda, even after a gruelling track session on the hottest U.K. day in decades.
(Tri Spark also helps with all of the above)
Installation is as easy as ABC.
And if you by a kit from cNw it comes correctly jetted and with a flow bench designed manifold.
Oh yes, and they liberate serious performance potential.

The only downside is they’re not cheap.

Some say the slides rattle. I say I’ve NEVER noticed that over the normal engine and peashooter noise!

Here’s mine when they were first fitted 5 yrs ago:

View attachment 4289

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Eddie you will burn in hell for fitting those carbs!!!!!
Although I have never fitted them myself I am here to tell you they are no good ha ha

Pistols at dawn Baz...

Fast Eddie said:

Pistols at dawn Baz...

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Seriously mate I'll give you a fiver for those carbs it'll save you all that grief you must be having with them!!!!!!

I’ll get back to you on that Baz.

Don’t hold your breath tho eh?!

Fast Eddie said:

Pistons at dawn Baz...

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Fixed the typo for you!

Not sure how to improve the top end of a single Mikuni on a Norton...but mainly not really sure why one would ever want to put a tortuous 2 in 1 manifold on a Norton head. Well...

Anyway, I run a pair of 32mm Amal concentrics on my Featherbed 850 with great success.
It is a stage 2 engine, with lightened rockers, flowed head, polished rods, PW3 cams, crank lightened and balanced for the frame.
No chokes fitted on the carbs. The bike starts first kick hot or cold (the bike sleeps outside), and holds a steady idle after less than 30 seconds.
I recently went back from velocity stacks to KN type filters, without any side effect under 110Mph. (The bike has been clocked at 213km/h - 130Mph on the Dijon circuit straight with the velocity stacks).

The things I did to improve the concentrics up to this sterling result are cheap and easy:

1) the carbs are rubber mounted and splayed (à la Triumph) thanks to...manifolds from a twin Mikuni kit! (See, I am open minded). Those are available from Allen Performance in the UK.

2) I use Amal MkII bronze slides (3 1/2)

3) I replaced the standard 106 needle jets by 107's while lowering the needle one notch from richest. This made the engine much 'fuller' below 4500rpm.

4) I run 260 main jets on the road and 290 on the track on Belgian 98 super petrol.

As I am of a curious nature, I am also busy rebuilding a pair of Dell Orto PHF 32mm 'pumpers' for comparison, I will post test results in due time.

Obviously, a pair of FCR 35 would most likely further improve my bike (my friend Yves use them and they are superb!), but I can't afford them.

triumph2 said:

Fixed the typo for you!

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Correction accepted!

Jagbruno, I have dyno’d velocity stacks back to back with K&N and foam socks and etc. They DO make a difference, but we are talking a few bhp. I think the biggest difference I ever saw was 4bhp, on a 900 Weslake, it’s usually much less.

So, for the obsessive, OCD, rain-mam types, by all means, go ahead and fit them. But for everyone else, there’s no need to bother cos the difference is undetectable when riding.

But I am one of the obsessive types! So I run velocity stacks inside a big foam sock. Best of both obsessive worlds !

1up3down said:

over and over I read people saying that a single carb, normally a Mikuni 34, cannot equal the horsepower performance of, for example, twin Amal Premiers because the single carb "cannot flow enough gas"

if that were true then simply raising the needle or fitting a larger main jet would richen the mixture, thereby flowing plenty of gas, but that does not work for more power

isn't the real issue the fact that a single carb set up cannot flow enough "air", not gas?

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Getting back to the original question, I think we have a language issue here. Over in England gas doesn't mean gasoline it mean gas as in air/oxygen/nitrogen etc.

I run two 34mm Mk2 Amals with tapered inlet ports. They have long velocity stacks which I believe don't do much except look good. However velocity stacks on two-strokes can kill performance. When Peter Inchley was playing with that Villiers crap, he used a GP carb with a long bell mouth - it would not rev out, until one day the velocity stack fell off and it revved straight up to 8000 RPM.
With my bike, I never get the situation where it becomes really sluggish due to over-richness because I use methanol fuel. But even then being even slightly rich on the needles takes the edge off the motor. With a single carb on petrol, you have got more hope of getting the jetting really right. How many of you guys have tried using different taper needles ? With petrol getting that area of the jetting right is critical. If you are jetted rich on the main jets but correct on the needles, you will usually still get good top end performance.
With velocity stacks - they really only work perfectly at one particular rev level, so they are a compromise. So whether they are short or long probably doesn't make much difference when you are riding over a variation in road conditions.