MPG- Twin Premiers vs single 34 Mikuni

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I think this stuff about twin carbs using more fuel because they make more power is a bit nonsensical. Half a thou of an inch wear in a needle jet when using petrol should take the edge right off performance - much more than the difference between twin or single carb.

Well...if you make more power you require more fuel to do it. Twin Amals can produce more power on the Commando than the typical single Mikuni. As I have experienced numerous times, a pair of Amals on an 850 will pull away from an 850 with single Mikuni and achieve a higher top speed. It will also, of course, use more fuel if you choose to exercise that capability.

I agree that the needle jet wear will make a fuel consumption difference while cruising but I think to make comparisons, we should be assuming that whatever carbs we're discussing are in proper, non-worn condition, correctly jetted, and have adequately sized, clean air filters.
 
I guess if you really want lots of mpg then fit an SU phoenix kit
People claim incredible mpg from those
If you can find one of course
 
If you are only changing one set of jets and keeping one air-cleaner serviced, you are much more likely to have your motor tuned to the optimum. I would not use a single carb for racing, but for a road bike, it might be better than twin carbs. I think this stuff about twin carbs using more fuel because they make more power is a bit nonsensical. Half a thou of an inch wear in a needle jet when using petrol should take the edge right off performance - much more than the difference between twin or single carb.

In order to make sense of this, one needs to understand that twin carbs will allow (or should I say are more amenable too) greater mass flow. This is a result of a combination of things that we will not get into here. Increase the mass flow rate of air and you need to increase the mass flow rate of fuel, really simple.

My personal preference is always staying with the twin carbs as 1.) I want that reserve power for certain driving conditions (that is why I gravitated to Norton Commandos), 2.) I am rarely (if ever) concerned with mileage, and 3.) I like to fettle.

I do understand the desireable attributes of a single carb.
 
I guess if you really want lots of mpg then fit an SU phoenix kit
People claim incredible mpg from those
If you can find one of course
I rode alongside a SU equipped iron head H-D Sportster, for 2,000 miles. That bike delivered over 50mpg, alongside my XS1100 at 35mpg. Super slab cruise, 75mph typical. Impressed me, to say the least.
 
I guess if you really want lots of mpg then fit an SU phoenix kit
People claim incredible mpg from those
If you can find one of course
I have one (not currently fitted) and it returned 70-75 mpg with excellent performance.
 
I don't particularly care about mileage. Others do. I have changed back and fourth between twin Amals and a single 34 Mikuni. There
doesn't seem to be much difference in mileage. I get 35-45 mpg depending on how aggressively I ride. I suppose if I putted along at
45-50 mph on flat pavement I could get 50 mpg. 20 tooth sprocket. Performance wise, I have seen little difference between the two until 4500 rpm when the twin Amals start to breath better and top speed is enhanced. While a different topic, it is quite apparent that the motor
is a much snappier performer with cool air temp and high humidity.
 
My experience with my 72 Combat and single 34mm Mk2 Amal for touring was up to 70mpg, whereas my current 850 with single Mikuni 34mm can only just achieve 50mpg.
Then again the two machines feel so very different in engine characteristics, there must be an efficiency with the higher performance of the 750 when being used gently?
 
As somebody already said
Fuel consumption does vary considerably depending on your throttle hand
I suppose in a perfect world with perfectly set up carbs/engine/ignition it shouldn't make much difference between a single slide carburettor or a pair of slide carburettors! Except maybe right up the top end??
I assume where the SU scores is the fact that it's a vacuum controlled slide?
I'm sure there are many out there that understands this black art
I unfortunately do not
 
As somebody already said
Fuel consumption does vary considerably depending on your throttle hand
I suppose in a perfect world with perfectly set up carbs/engine/ignition it shouldn't make much difference between a single slide carburettor or a pair of slide carburettors! Except maybe right up the top end??
I assume where the SU scores is the fact that it's a vacuum controlled slide?
I'm sure there are many out there that understands this black art
I unfortunately do not
It's not complicated, the throttle opens a butterfly valve then the slide opens according to how much vacuum the engine is sucking allowing the exact amount of gas/petrol required. Even my old grandmother understood that. ... . . . . Some women know how to do b*** jobs!
 
If both set-ups are tuned optimally and you are judicious with throttle movement while cruising at a steady state the twin Amals could have the edge. The Amals have the potential to use more fuel with almost twice the cross sectional area, but as mentioned earlier it takes the same amount of A/F mixture to develop the same power. I believe the intake tuning of the Amals is better than the 1 into 2 Mikuni manifold, but the simplicity of the single carb is likely to insure a more uniform distribution of the A/F mixture and doesn't need to be synced; out of sync twin carbs can cause one cylinder to run lean and as also pointed out slide ware throws another unknown into the pot.

Tough choice. If you choose to go with the single Mikuni PM me and we can discuss jetting, most of the kits out there are jetted too rich.

Best
 
It's not complicated, the throttle opens a butterfly valve then the slide opens according to how much vacuum the engine is sucking allowing the exact amount of gas/petrol required. Even my old grandmother understood that. ... . . . . Some women know how to do b*** jobs!
Yep I understand how the vacuum carburettor works
What I meant was there are people that understand carburetion better than me
The best set up I have ever had on my 750 is a 40mm TM mikuni flatslide
I have no idea why it performs so well
I'm thinking a flatslide mixes/atomises the fuel better than a round slide carb?
Or maybe it's the pumper helping here?
 
Yep I understand how the vacuum carburettor works
What I meant was there are people that understand carburetion better than me
The best set up I have ever had on my 750 is a 40mm TM mikuni flatslide
I have no idea why it performs so well
I'm thinking a flatslide mixes/atomises the fuel better than a round slide carb?
Or maybe it's the pumper helping here?
Ever met some idol (idiot ) who put the round type slide in back towards front? It happened to me when I took a 350lc into a m/c shop for repair and I did 60 miles on 3 gallons of fuel. And you and others say flat slide carries work? Now that is a big mystery to me.
 
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I have run my 850 with the 22T for 32,000 miles, in Smokey, White, Appalachian Mountains. A lot of it with saddlebags & my 240lbs., no wish for different gearing, ever. I chose the 22 because I like long legs, not tractor gears.
That said, it’s hard (unsafe)to go 65 when traffic is rolling 80. I’ve seen 50 mpg (twin Amals) when on a Parade ride. Low 30’s at 80mph.

This is the second highest pass in California. A couple of passes in Colorado are higher.
 

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I read a lot of good answers here. There are many factors: wear, internal design, both in the carb and engine factors, and environmental. I remember driving (driving not riding) west once in an old TR-4. It had started out in fine shape when I left Pittsburgh, but somewhere west of the Panhandle, it started requiring more and more pedal (gas). It got to the point where it was almost floored at 60 miles per hour. And getting worse. I just kept praying that I would make it to a gas station. Few and far between out there then. Some years before the completion of the interstates. But nothing was improving, so I pulled over and pulled the hand brake to go out and look under the hood while it was running. But I could hardly open the door! I had to squeeze out actually, the wind was blowing so hard. There was no indication, no trees with all their limbs horizontal, no grass flattened over. Just rocks and the wind at God knows what velocity. The Triumph seemed to be idling just fine and responded quite well when I blipped the throttle. "Wow," I thought, and started off again. The wind died down by night fall. So I really don't know how to asses all those factors, even from ride to ride, but only in a general sense as has been mentioned. With such a wandering preamble I would think the experience of even two different people on the same bike cruising around at the same speed would produce some difference in results. As for the SU's with vacuum activated slides, they might account for altitude which could be a factor even for small changes. Well, my ears pop even going up and down moderate hills in western Ohio. Don't know to what degree that affects air fuel mixture, but maybe with greater altitude changes.
 
I read a lot of good answers here. There are many factors: wear, internal design, both in the carb and engine factors, and environmental. I remember driving (driving not riding) west once in an old TR-4. It had started out in fine shape when I left Pittsburgh, but somewhere west of the Panhandle, it started requiring more and more pedal (gas). It got to the point where it was almost floored at 60 miles per hour. And getting worse. I just kept praying that I would make it to a gas station. Few and far between out there then. Some years before the completion of the interstates. But nothing was improving, so I pulled over and pulled the hand brake to go out and look under the hood while it was running. But I could hardly open the door! I had to squeeze out actually, the wind was blowing so hard. There was no indication, no trees with all their limbs horizontal, no grass flattened over. Just rocks and the wind at God knows what velocity. The Triumph seemed to be idling just fine and responded quite well when I blipped the throttle. "Wow," I thought, and started off again. The wind died down by night fall. So I really don't know how to asses all those factors, even from ride to ride, but only in a general sense as has been mentioned. With such a wandering preamble I would think the experience of even two different people on the same bike cruising around at the same speed would produce some difference in results. As for the SU's with vacuum activated slides, they might account for altitude which could be a factor even for small changes. Well, my ears pop even going up and down moderate hills in western Ohio. Don't know to what degree that affects air fuel mixture, but maybe with greater altitude changes.
The SUs would if taken up high attitudes be adjusted to a weaker setting by a perfectionist and who would do the same again making mixture richer coming back down again. Older aircraft have a mixture control setting in the cockpit .
 
The SUs would if taken up high attitudes be adjusted to a weaker setting by a perfectionist and who would do the same again making mixture richer coming back down again. Older aircraft have a mixture control setting in the cockpit .

Other way around.
 
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