One Into Two Manifolds

in general but more specifically the single Mikuni manifold handling the mixture flow into our commando heads

I have read various automotive articles that the manifold itself can be improved to better flow at higher rpm

my understanding is that around 4500-5000 rpm and higher on Commando motors the single Mikuni cannot flow enough air as a twin Amal set up because of the interior elbow blocking the flow and that at higher rpm's a certain amount of gas droplets don't make it into the combustion chambers, dropping to the bottom of the manifold

comments on automotive forums give the impression that a commercial grade manifold can be professionally machined to better deal with this higher rpm issue

is this correct, can anything worthwhile be done to our Mikuni manifolds, how, anyone doing this?

I think the problem is that the carb is too close to the head, necessitating tight bends.

If so, the actual making of a new manifold isn't the issue, it's making space to move the carb further away from the head.

As Fast Eddie said, the carb is too close to the head to be able to make big improvements, but I have made mild improvements in a standard MAP manifold.

Think of it this way, air likes to move in straight lines -not follow curves.

If you look through a 2 into 1 manifold you will see there is a limited amount of area where you can see straight through it. If you remove material from the places where it blocks your vision you will end up with better flow.

Simply using a 38mm manifold with a smaller carb on an adapter ring will increase the performance over the smaller manifold.

The carb size itself has little bearing on the maximum power but the smaller carb will be easier to make work well at lower rpm. Jim

Single mikunis unless a tm40 make a commando gutless in my opinion,I've had a single 32cv a 34vm and a 36vm on my commando could hardly tell the difference between them,but when I fitted the 40 it made a huge difference ,I would not recommend the single vm to anyone , cheers

'Think of it this way, air likes to move in straight lines -not follow curves.'

Particularly with two strokes, much of the noise pollution comes from the intakes. That indicates that what is happening is at sonic speeds. Flow benches don't usually operate at sonic speeds so probably don't accurately predict what happens when motors are running at high revs. I've used both separate carbs and single carbs with manifold on Triumph 650s for road use and noticed no difference. However for racing I always use separate carbs with long bell mouths.

yes, I too used to be of the opinion that a single Mikuni was "gutless" compared to two Amals

until I happened to read a March of 1975 article published in Cycle Magazine, the Mikuni Conversion

the article detailed both the easy Mikuni installation retaining the original air filter but also its performance

Cycle fitted a Commando first with a good new set of Concentrics and compared it at the track to a similar Commando fitted with a 34mm single Mikuni

they found that the Mikuni equipped Commando out accelerated the twin Amal bike in the typical 50-80mph range,
in fact by four bike lengths, and raved about the superior gas less tickling starting and idling.

they found that the twin Amals delivered more horsepower only from about 5000rpm on up not because they could flow more gas but could flow the mixture straighter into the combustion chambers

in the quarter mile acceleration testing the twin Amal equipped bike was 3-6 TENTHS of second quicker
or 13.2 versus 13.7 for the MIkuni bike, hardly a "gutless" comparison under real world testing

The Cycle Magazine article is available as published in the book I have of all the road tests of the era
the book is titled Norton Commando - Ultimate Portfolio, and the Mikuni conversion reprint is on page 144

While I might agree that a correctly jetted single Mikuni may outperform a set of stock, poorly jetted Amals,
I would not agree that a single Mikuni on a MAP manifold will outperform a pair of correctly jetted Amals at any rpm. I have proved it several times on my dyno. Jim

I put a TM, not VM, 34mm on my bike with the bigger MAP manifold Jim recommended. This is a flat slide carb, maybe it flows better than the VM, but I promise I noticed NO drop in acceleration at all. This plus the reliable starting with no drooling makes me very happy with the setup.
Jaydee

It's basically the better mixture control that makes a Mikuni outperform Amals.

It can be done with dual Amals but it isn't easy or long lasting. You would start out by drilling the needle jet air passage and installing an air jet and then modifying the slide to use Mikuni needles. Then with some dyno time with a gas analyzer, you may get there.

If you want better performance at any rpm than either a perfectly setup set of Amals or a single Mikuni
then bolt on a pair of Mikunis. They will make better power all the way from idle to flat out. Too bad they don't fit
very well in a Commando frame. Jim

While I might agree that a correctly jetted single Mikuni may outperform a set of stock, poorly jetted Amals,

Click to expand...

the Cycle Magazine article I quoted was very clear that they considered Amals losing their effectiveness around 5000 miles, and thus made sure that they fitted a very good set to the test Commando so that the validity of their comparison to the Mikuni would not be questioned for that reason

the reason I am bringing up this Cycle reprint is that it is the only one I could find published by a credible testing source

I do understand that people may disagree based on their own instinctual bias or just plain guesses, and for that reason
I would very much like to see an actual credible test result that contradicts the Cycle Magazine's, but until then........

1up3down said:

While I might agree that a correctly jetted single Mikuni may outperform a set of stock, poorly jetted Amals,

Click to expand...

the Cycle Magazine article I quoted was very clear that they considered Amals losing their effectiveness around 5000 miles, and thus made sure that they fitted a very good set to the test Commando so that the validity of their comparison to the Mikuni would not be questioned for that reason

the reason I am bringing up this Cycle reprint is that it is the only one I could find published by a credible testing source

I do understand that people may disagree based on their own instinctual bias or just plain guesses, and for that reason
I would very much like to see an actual credible test result that contradicts the Cycle Magazine's, but until then........

Click to expand...

Standard Amal carbs -new or not - are going to be rich at mid rpm under heavy throttle, where the single Mikuni may out-perform them.

Most any carb will come with this designed into them to avoid engine damage. They most certainly would not have a carb setup that was anywhere near close to optimum on a Norton because of the rather wide variations seen from one engine to the next.

Unfortunately the available jets and needles for an Amal will not correct this, so I would doubt that Cycle magazine did anything beyond a main jet check. They most certainly did not have the equipment available to correctly calibrate one for mid range performance.

And no, I am not going to start posting dyno results. Anyone who has been around a dyno very long knows just how worthless dyno slips are. They are best used only as a comparison tool for the tuner. Jim

I made my own inlet manifold to suit the TM carb ,I built up a wedge/steeple in between the inlet ports where the central manifold mounting screws would have been ,the TM flatslide completely out performs the VM in all areas in my opinion, especially at the top end,my bike will carry on into the red if you want it to,also if you spend time above 90mph it won't drain the float bowl like VMs do , biggest issue for most people would be that you really need to cut the gusset out of your frame to get it to fit right, cheers

Baz, what are the options for fitting the carb further away from the head, beyond the gusset? (battery re-positionned)

gripper said:

Baz, what are the options for fitting the carb further away from the head, beyond the gusset? (battery re-positionned)

Click to expand...

I suppose you could but that would be one hell of a long manifold,I only tried this out to see for myself what it would do,I was quite surprised how well it went!! I was expecting it to run like a VM ,as a complete novice at this stuff I can only imagine a flatslide atomises the fuel better?? And the reason I put a wedge inside the manifold was to deflect the incoming charge into the inlet ports,and stop the turbulence created between the port's, whether this works or not i do not know

One of the best running single carb setups I did several years ago was a long intake manifold which consisted of a flange with hose spigots on the head connected to a Y shaped piece with a couple sections of curved radiator hose.
The manifold length placed the carb mounting flange about even with the frame web.
I mounted a 42mm Keihin Harley carb of this style.



It was a surprising performer. It did require cutting out the web but the carb was short since it is not a slide carb so there was no interference with the backbone tube. The cable operated choke and accelerator pump made starting very easy. Jim

jaydee75 said:

I put a TM, not VM, 34mm on my bike with the bigger MAP manifold Jim recommended. This is a flat slide carb, maybe it flows better than the VM, but I promise I noticed NO drop in acceleration at all. This plus the reliable starting with no drooling makes me very happy with the setup.
Jaydee

Click to expand...

Any fitment issues? Do Mikuni make an adapter to fit the TM34 to the 38mm manifold?

No fitment issues. I have a lathe so all I had to do is make an aluminum donut bushing to slip over the carb mouth and fit tightly into the manifold adapter.

https://i.screenshot.net/8r88ltz

https://i.screenshot.net/rxnn2ak

Jaydee

so if I had a 36 mm carb which manifold should i use 38? plus adapter ring

chrisshropshire said:

so if I had a 36 mm carb which manifold should i use 38? plus adapter ring

Click to expand...

This,
http://www.rockypointcycle.com/p-norton ... -2535.html
And this,
http://www.rockypointcycle.com/p-norton ... -2565.html

sorry I meant to ask if I had a 36 mm carb should I get a bigger map manifold ? 38 or 40 mm and make up an adapter ring
as lots of folks say that the single carb works better with the larger manifold