Twin Carbs versus Single Mikuni 36mm (2012)

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1up3down said:
Dave Dregens did this on his Manx featherbed Triumph twin in the Barcelona winning bike back in the 1960s when he beat the works Honda 750sthere was a story of it somewhere ……..
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Ok, so one guy some 45 years ago was competitive with a single carb set up. I stand corrected.

And no doubt someone will point out another example

But seriously, from dyno testing to decades of race tests, it is clear that twin carbs flow more air at higher rpm and as such create more horsepower and top speed than all things being equal single carbs.

However, wishful thinking and desire to be factually contrary for the sake of being different can argue otherwise.
Click to expand...


I'm the twin carb camp. For example, a GREAT BIG single carb would work well at high speed, like a VM44, (I've run TWO on a 540cc two-stroke, but that's another story) but would run very poorly at low speed on these engines. The beauty of two smaller carbs is they can do both, work well at low speed and flow enough at high speed. Simple, huh? The big issue here is, if the "black art" of carburetion has made a monkey of you, then a single Mik may be your salvation. If carbs and their theory don't intimidate you.. twins work wonderfully.
 
Not so :!:
Dave Dregens did this on his Manx featherbed Triumph twin in the Barcelona winning bike back in the 1960s when he beat the works Honda 750sthere was a story of it somewhere …….. :)[/quote]


Yes, that is factually correct, but it was a 24 hour race on a very tight twisty circuit, and the motor was detuned to cope with the lower quality petrol available, and to improve fuel consumption thereby reducing fuel stops during the race (where power was at a lower premium than handling and reliability
 
Diablouph said:
I've always just used the cables for idle adjustment and never had any problems. If the cables are adjusted properly all the idle screw does is create slop in the cable.
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For you, that may work, but many bikes have cables which move around as you steer and the tighten the cable.
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You could edit that to say "all bikes (with carbs)" Don't forget, the inner and outer cables mostly move together unless you have a routing problem. Besides, the throttle stop screw is only in use at idle and there isn't much steering going on then. People have a tendency to try to keep all cables, wire and control cables tight and neat looking by gathering them close to the handle bars till they disappear into the tank tunnel. Control cables should be in long mild curves without too many sharp bends which create tension and friction between the inner and outer cables. All this does is create extra work for the throttle return spring until it weakens it, slowing down it's reaction and preventing full closing. The fact that Amal is manufacturing carbs without a throttle stop screw doesn't bother me. With proper idle circuit settings and proper slides it shouldn't be needed.
 
The other thing that concerns me about single carbs is not just the shape of the manifold and it's flow characteristics but the fact that during the intake stroke the other cylinder is on the power stroke and after combustion valves are not fully closed at the end of the stroke specially with stronger cams. Some of the exhaust gasses escape into the intake port and can effect the fuel air mixture beginning to be drawn into the opposite cylinder at the end of it's intake stroke. These gasses can disturb the flow of incoming gasses as well as dilute it. On an exhaust side of things this situation is well taken care of with collectors used in multi cylinder engines. The volume of expelled gasses is given space to occupy before it collides with the suction on other cylinders caused by valve overlap so exhaust gasses are not brought back into the combustion chamber of other cylinders. Most 2 into 1 exhausts used on bikes account for this in the length of pipe before they join and actually work to keep exhaust gasses moving out while creating suction in the other cylinder, aiding intake of fresh fuel air mix through the still closing exhaust valve of that cylinder. On the intake side of things the intake manifolds on engines like the commando are so short there can be little allowance for this conflict of gasses during valve overlap. Single carbs would eliminate this issue between cylinders. No matter how carefully valve timing is plotted out during cam design these inefficiencies exist and create some degree of disadvantage to single carbs not seen in dual carbs.
 
Synchonious parambulations ;
Quote :
What did more was the single carburettor bike that I built for my return to Barcelona in 1970 when I won again. It was just common sense really; no point in lots of fuel stops and high power for a hillside circuit with seven hairpins downhill, sometimes and only one straight What you want is surge coming out of comers.

Furva to and insofar ;

http://www.dresda.co.uk/ljk.asp
 
Rip van nailed the main reason hi perf. street engines prefer single intakes to each jug, to avoid manifold cross suctions stealing flow and getting too much fuel. Again 360' twins have the least issue with this so its not the main down side for single carb on them.

So with the basics covered, which would be better on the Cdo in question,
Twin 34's or a 36 single? Twin 34's vs a 38 pumper? Twin 34's pumpers vs a plain 36 or 38? Better in in town or canyon endurance runs vs Daytona or salt lake?

Buy Jim's 175 hp capable big single throat Y manifold injection system and dial in for anything?
 
RennieK said:
The other thing that concerns me about single carbs is not just the shape of the manifold and it's flow characteristics but the fact that during the intake stroke the other cylinder is on the power stroke and after combustion valves are not fully closed at the end of the stroke specially with stronger cams. Some of the exhaust gasses escape into the intake port and can effect the fuel air mixture beginning to be drawn into the opposite cylinder at the end of it's intake stroke. These gasses can disturb the flow of incoming gasses as well as dilute it. On an exhaust side of things this situation is well taken care of with collectors used in multi cylinder engines. The volume of expelled gasses is given space to occupy before it collides with the suction on other cylinders caused by valve overlap so exhaust gasses are not brought back into the combustion chamber of other cylinders. Most 2 into 1 exhausts used on bikes account for this in the length of pipe before they join and actually work to keep exhaust gasses moving out while creating suction in the other cylinder, aiding intake of fresh fuel air mix through the still closing exhaust valve of that cylinder. On the intake side of things the intake manifolds on engines like the commando are so short there can be little allowance for this conflict of gasses during valve overlap. Single carbs would eliminate this issue between cylinders. No matter how carefully valve timing is plotted out during cam design these inefficiencies exist and create some degree of disadvantage to single carbs not seen in dual carbs.
Click to expand...


Will reed valves on each port stop this :?:
 
concours said:
1up3down said:
Dave Dregens did this on his Manx featherbed Triumph twin in the Barcelona winning bike back in the 1960s when he beat the works Honda 750sthere was a story of it somewhere ……..
Click to expand...

Ok, so one guy some 45 years ago was competitive with a single carb set up. I stand corrected.

And no doubt someone will point out another example

But seriously, from dyno testing to decades of race tests, it is clear that twin carbs flow more air at higher rpm and as such create more horsepower and top speed than all things being equal single carbs.

However, wishful thinking and desire to be factually contrary for the sake of being different can argue otherwise.
Click to expand...


I'm the twin carb camp. For example, a GREAT BIG single carb would work well at high speed, like a VM44, (I've run TWO on a 540cc two-stroke, but that's another story) but would run very poorly at low speed on these engines. The beauty of two smaller carbs is they can do both, work well at low speed and flow enough at high speed. Simple, huh? The big issue here is, if the "black art" of carburetion has made a monkey of you, then a single Mik may be your salvation. If carbs and their theory don't intimidate you.. twins work wonderfully.
Click to expand...

To 1up3down;
One carb on each cylinder is the best set up for top end power, of that there is no question
As I have stated elsewhere on this forum the single carb setup has its use, by delivering engine power lower down the rev scale.
I had an ex-police Atlas with a single Monoblock carb that would pull from 20-25mph in top gear.

To concurs;
You hit the nail right on the head, the engine will run poorly at low speed with oversize carbs, so, they are partially useless outside of the race track, as always, the manufactures’ of engines aim for a compromise as to getting the best possible low speed combined with a reasonable high speed.
With a single carb setup you nearly ALWAYS sacrifice the top speed.
Its swings and roundabouts, you pay your money and take your choice :!:
 
Matt Spencer said:
You dont have that problem if you supercharge it .
Click to expand...

Yea, right, and if you don’t declare that you have supercharged the engine when you take out your insurance, it’s nil & void.
 
A single carb works well in the low to mid RPM range. But the bowl runs dry if held at WOT unless you go to such a large carb that it may no longer run well in the low & mid RPM range. That doesn't matter to those who don't need sustained top speed. But some occasionally feel the need for the top end rush.
 
jseng1 said:
A single carb works well in the low to mid RPM range. But the bowl runs dry if held at WOT unless you go to such a large carb that it may no longer run well in the low & mid RPM range. That doesn't matter to those who don't need sustained top speed. But some occasionally feel the need for the top end rush.
Click to expand...
This is the generally accepted and the logical explanation and I wish not one to disagree so I won't.
I will say this. There seems a solution to every need and exceptions to every rule.

My 40mm will pull stumps from below 3000 to 4200 AND THEN it decide to kick in. Reaching red line is effortless, float bowl does not run dry when held wot for extended periods of time and the need for an ignition system with rev limiter or JIMs beehive valves springs is apparent.
This is not a sales pitch I do not recommend this scenario for the lite hearted or the stock norton (stock Combat maybe) All I am saying is the there no absolutes and we are limited only by our imagination, our willingness to indulge and our monetary condition.

Hey! How about a pair of MK2's. That's what many racers are using. Maybe I'll see how that goes. They have new adapters that bring the carb right up close to the head, although I would like to apply a little down angle.
Food for thought.
 
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