convert back from mikuni to 2 amals

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after a nice talk this afternoon with matt at Colorado Norton Works I have decided to retain my single 34mm mikuni based on my riding style (slow). he is a great guy and an honest businessman who could have sold me products but gave advice based on my needs...he is a true gentleman. I have decided to take the $$ I was going to spend on this conversion and give it to a local shelter where there are a number of homeless families with children.
 
That's very kind of you. Not to hijack the thread but why do so many hate the twin Amals? I have them on my '72 Triumph Daytona and my '73 Norton and don't mind them. Fairly new guy here.
 
Eric Bauer said:
That's very kind of you. Not to hijack the thread but why do so many hate the twin Amals? I have them on my '72 Triumph Daytona and my '73 Norton and don't mind them. Fairly new guy here.
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I think because of when the normal Amally challenged bikes were daily drivers, in post war UK they were made with trash metal and wore out quickly, thus leading to poor running.

Anodized slides, stay up floats, cleanable idle circuits all make a difference.
 
Eric Bauer said:
That's very kind of you. Not to hijack the thread but why do so many hate the twin Amals? I have them on my '72 Triumph Daytona and my '73 Norton and don't mind them. Fairly new guy here.
Click to expand...

That sir is a full blown hijack right there... this one should run for a bit...!

There are two main groups in this one, twin Amal’s and single Mikuni. And both groups will tell you (truthfully) that their carb runs sweet.

I had a 36mm Mikuni on my Commando and it worked well, but it clearly restricted it at anything over 5,000rpm. Of course, for many people, performance above 5,000 is just not relevant anymore.

If you think about it, not many people replace brand new perfectly tuned Amal’s with a Mikuni. What they (usually) do is replace old, worn out, badly tuned Amal’s with a brand new Mikuni. In such cases the Mikuni is bound to ‘transform’ their bike, and that will probably create an Amal hater !

My Mikuni ran sweet below 5,000. But then again, I have twin Amal’s on other bikes, and they run sweet too.

IMHO one of the worst things you can do with twin Amal’s is have a twin pull twist grip. The cables do seem to easily and frequently go out of adjustment. Having two short cables to a junction box and then one cable to the twistgrip definitely reduces, in fact almost eliminates, these ‘going out of balance’ issues.

Original Amal’s were definitely made to a price (as posted above). Some claim they’ll be worn out at 10,000 miles. Modern Premieres address most of these issues. Mikunis are very nicely made, hard chrome slide as standard etc.

Generally speaking, Amal’s are easier to tune. They are also far more tolerant of imperfect tuning, dirty fuel etc. Mikuni’s can offer more precise metering WHEN set up properly (Fullauto used an exhaust gas sensor to set his up, so his will be as good as they get, but most folk don’t do this, so are most likely running with compromises).

Currently I’ve got one Brit with Mikunis, one with Keihin flat slides, and two with Amal’s... so I am forced to try and remain objective !
 
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When I sleeve the amals most want to retain the performance otherwise the the single mikuni seems to work fine for riders not caring so much for the top end performance.When I get the amals they are totally clapped out.It is a big investment to replace them new as well as the mikuni conversion so I guess there is no hard and fast rule.Depends on your riding style.
 
I suspect that the original Concentrics will work well for longer than generally given credit for. In my limited experience it seems that with a couple of needle jet replacements and functioning air filters one can get about 30,000 miles down the road before the slides are so worn that they just keep taking out the needle jet.
My Vincent had Concentrics with higher mileage than that and very loose slides.
It ran fine but started to burn a tremendous amount of fuel which meant the needle jets were worn out ( again). I changed them a couple of times .Each time fuel economy came right back to normal 55mpg. Finally I decided to renew with Premieres, not a bad choice.
The Commando has 26,000 miles on it now and it runs very well, original Amals with recent needle jet replacement, first time on that bike.

Glen
 
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I’m older and lazy , 2 bullet proof Italian bikes to go far and fast on , Mikuni on my Commando has worked fine (since ‘04) for my Sunday rides ....
 
tri-man said:
A couple of years ago a customer asked me if I could fit his two 930 Amals off of a Bonneville to his RH4 head and toss the single 34mm Mikuni.
I sleeved the ports to 30mm, got a pair of 750 inlets that I opened up to 30mm on the inlet side, and went up to (from memory) 250 main jets. 106 needle jets and standard 2-ring needles. It worked really well and gave an eminently tractable engine with a marvelous throttle response. If it gave away anything up top compared to a 932 equipped bike I couldn't say, but it certainly went well.
A friend with a tuned 810 Dunstall kit, big valves and cams couldn't believe it, one of the quickest Nortons he'd ever ridden he said.
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There is no question, going up to the 32mm port with the same 1.5 inch valve was a bad move.

The small valve limits the airflow so the large port ends up with low velocity and poor cylinder filling efficiency.

The carb size doesn't make a big difference, it will just maintain the torque to a little higher rpm with the 32's and a tapered manifold. But it does loose a little throttle response down low with the 32's.
 
comnoz said:
There is no question, going up to the 32mm port with the same 1.5 inch valve was a bad move.

The small valve limits the airflow so the large port ends up with low velocity and poor cylinder filling efficiency.
Click to expand...

So does a bigger valve restore the equilibrium Jim?
 
Fast Eddie said:
So does a bigger valve restore the equilibrium Jim?
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I helps a lot but, the 32mm port is not really well shaped for high velocity flow. The short side radius is too small. [unless you add an insert or sleeve]

Better results can be had by reshaping a 30mm port and adding a big valve.
 
If I remember correctly - a bit questionable :) - historically it was said that fitting 32mm carbs to a 30mm inlet
left a "step" that created turbulence which was said to be beneficial. Is that a load of spherical dangly bits?
(No emo thingy for dangly bits)

Just remembered the carb mounts are tapered 32mm to 30mm. But I still have a vague memory of reading
about the 32/30mm step causing beneficial turbulance:(
 
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Fast Eddie said:
That sir is a full blown hijack right there... this one should run for a bit...!

There are two main groups in this one, twin Amal’s and single Mikuni. And both groups will tell you (truthfully) that their carb runs sweet.

I had a 36mm Mikuni on my Commando and it worked well, but it clearly restricted it at anything over 5,000rpm. Of course, for many people, performance above 5,000 is just not relevant anymore.

If you think about it, not many people replace brand new perfectly tuned Amal’s with a Mikuni. What they (usually) do is replace old, worn out, badly tuned Amal’s with a brand new Mikuni. In such cases the Mikuni is bound to ‘transform’ their bike, and that will probably create an Amal hater !

My Mikuni ran sweet below 5,000. But then again, I have twin Amal’s on other bikes, and they run sweet too.

IMHO one of the worst things you can do with twin Amal’s is have a twin pull twist grip. The cables do seem to easily and frequently go out of adjustment. Having two short cables to a junction box and then one cable to the twistgrip definitely reduces, in fact almost eliminates, these ‘going out of balance’ issues.

Original Amal’s were definitely made to a price (as posted above). Some claim they’ll be worn out at 10,000 miles. Modern Premieres address most of these issues. Mikunis are very nicely made, hard chrome slide as standard etc.

Generally speaking, Amal’s are easier to tune. They are also far more tolerant of imperfect tuning, dirty fuel etc. Mikuni’s can offer more precise metering WHEN set up properly (Fullauto used an exhaust gas sensor to set his up, so his will be as good as they get, but most folk don’t do this, so are most likely running with compromises).

Currently I’ve got one Brit with Mikunis, one with Keihin flat slides, and two with Amal’s... so I am forced to try and remain objective !
Click to expand...

That answers my question - sorry for the full blown hi jack :)
 
Staytite said:
If I remember correctly - a bit questionable :) - historically it was said that fitting 32mm carbs to a 30mm inlet
left a "step" that created turbulence which was said to be beneficial. Is that a load of spherical dangly bits?
(No emo thingy for dangly bits)

Just remembered the carb mounts are tapered 32mm to 30mm. But I still have a vague memory of reading
about the 32/30mm step causing beneficial turbulance:(
Click to expand...

Dunno about the Norton, but in the automotive world that's a no-no. At some flow level you get a reversion effect. That's what die grinders were made for.
 
Staytite said:
If I remember correctly - a bit questionable :) - historically it was said that fitting 32mm carbs to a 30mm inlet
left a "step" that created turbulence which was said to be beneficial. Is that a load of spherical dangly bits?
(No emo thingy for dangly bits)

Just remembered the carb mounts are tapered 32mm to 30mm. But I still have a vague memory of reading
about the 32/30mm step causing beneficial turbulance:(
Click to expand...
The story went something like a 350 goldstar was being tested on a Dyno at the BSA factory
An apprentice accidentally fitted a carb from a 500 to the 350 and on the Dyno it showed a sudden jump in power because of the step in the inlet port causing turbulence!!!
Often heard this story but if any of it is true is another question!!
 
Well how about that
Wrong bike make, wrong number of cylinders, but sort of correct memory.
Difficult to believe step in inlet gives beneficial turbulence.
Glass of Prosecco with my cornflakes
 
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Air can definitely do unexpected things. A couple of things I have noticed.

With a head on the flow bench and a 34 mm single carb manifold mounted, the flow is miserable- as expected.
But if you take the bolts out and hold the manifold against the head nothing changes until you slide the manifold about 1/8th inch to the side so the ports are mis -matched, the airflow increases by ~15% on the side you slide the manifold toward, and drops on the other side.

Another mystery,
With a head on the bench with an exhaust port flowing with a 24 inch exhaust pipe with a 90 degree angle a few inches from the port, similar to what you would see on a motor.
If the pipe is pointed down like you see with low pipes you get one number.
But if you rotate the pipe so it is pointed to the side, like with SS pipes, the flow increases considerably.
This shows up on the dyno also.
 
comnoz said:
Another mystery,
With a head on the bench with an exhaust port flowing with a 24 inch exhaust pipe with a 90 degree angle a few inches from the port, similar to what you would see on a motor.
If the pipe is pointed down like you see with low pipes you get one number.
But if you rotate the pipe so it is pointed to the side, like with SS pipes, the flow increases considerably.
This shows up on the dyno also.
Click to expand...

This is very interesting. I have one of Mike's 2EX1 exhausts, and the right pipe takes a sharp bend across the front if the barrel while the left one extends out before bending down.

There is quite a difference in how the Amals are jetted to get a similar plug color. The right carb has a larger needle and main jets.

I had wondered if it was a porting difference, or something else.
 
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