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Dual Carb Vs. Single Carb - Not A Tdm Question


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#1 Pjkr44

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Posted 16 March 2019 - 11:55 pm

Hello fellas.

It's not about tdm, more a general question: I've read some bikes have dual carbs per cylinder and this came as a surprise, I did not know that this set up exists. Did some research, but there's a lot of guessing out there...
Can anyone with real knowledge explain the difference, pros and cons, which set up better for what situation. Much appreciated.

#2 Favs

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Posted 17 March 2019 - 07:45 am

Gilera Nordwest 600 has this. Lots of info online.


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#3 wicklamulla

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Posted 17 March 2019 - 10:06 am

XT350 has 2 carbs,  They are quite different to one anudder too.  


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#4 Studley Ramrod

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Posted 17 March 2019 - 01:12 pm

My understanding, from watching various tv proggys related to mechanical stuff, mighta been an episode of Wheeler Dealers with Ed China, is that one carb controls the fuel flow at lower revs and the other carb kicks in at higher revs to provide more juice.  I also think it's quite dated in design.  No idea why it is/was used, or if there are any benefits to using this design though.


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#5 Pjkr44

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Posted 17 March 2019 - 10:50 pm

I came across while reading about Suzuki Freewind, that's got a double carb.. kind of makes sense if power delivery is more widespread, just more hassle.
Thanks fellas

#6 TKH

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Posted 18 March 2019 - 10:59 am

My understanding, from watching various tv proggys related to mechanical stuff, mighta been an episode of Wheeler Dealers with Ed China, is that one carb controls the fuel flow at lower revs and the other carb kicks in at higher revs to provide more juice.  I also think it's quite dated in design.  No idea why it is/was used, or if there are any benefits to using this design though.

 

I had a few cars in my younger days which had twin carbs. If I remember rightly these were referred to as twin choke. But they fed all 4 or 6 cylinders rather than 2 per cylinder. I never noticed when fuel was being introduced by the second carb but maybe that meant it was balanced ok. This was the sort of thing with, e.g., weber carbs. Sorry I digress. Although can't see why the idea wouldn't work for one cylinder. It'll dump a lot of fuel in.

 

After a quick look I saw this:

https://www.youtube....h?v=Ms7HA9w6QyE

 

I tried a quick google and there isn't much definitive information out there.



#7 Studley Ramrod

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Posted 18 March 2019 - 03:12 pm

A twin choke carb refers to the number of choke's each carb has, loike twin choke webers, one carbs with 2 chokes/butterflys, a dual carb setup is related to the number of carbs per cylinder.  As in, having 2 separate carbs for each cylinder. :)  Which is prolly why Pj is asking why use a dual carb setup, esp. when ya can get a twin choke carb that prolly does exactly the same thing.


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#8 Pedro

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Posted 19 March 2019 - 02:46 pm

I had a '71 Honda Minitrail a few years ago. I put two carbs on it splicing the original inlet with original carb to an aftermarket bigger inlet with bigger carbs.  I had a thumb throttle for the smaller carb and gave me an extra 500rpm's when flat out! I got lucky with the jetting as it worked perfect straight away.  And the nitrous helped :rolleyes:

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#9 Studley Ramrod

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Posted 19 March 2019 - 04:27 pm

That's a cool little bike ! :lol:  :good:


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#10 fixitsan

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Posted 19 March 2019 - 04:29 pm

The problem is when we try to get best performance over a whole range of operating parameters we soon find it isn't easy with a single carb. To get absolute peak power you need a wide unrestricted choke with a 'fat' jet to get maximum fuel/air into the cylinders at full speed. At full speed a lot of air is being moved so the velocity through the carb is high and the fuel atomises well

 

At the other end of the speed range, assuming the same wide inlet choke above, airflow is relatively slow and the  fuel doesn't atomise well, and the result is low torque output. A smaller choke means faster airflow and better atomisation at lower engine speeds.

 

A double carb lets you manage both situations, giving a smaller single choke for low rpm's, high air intake speed with good atomisation = good torque...but when you open it up the limiting factor would be the small inlet choke diameter, so a second choke opens up to allow much more air through and as the revs continue to rise, a good high rpm power output.


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#11 Studley Ramrod

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Posted 19 March 2019 - 04:35 pm

The problem is when we try to get best performance over a whole range of operating parameters we soon find it isn't easy with a single carb. To get absolute peak power you need a wide unrestricted choke with a 'fat' jet to get maximum fuel/air into the cylinders at full speed. At full speed a lot of air is being moved so the velocity through the carb is high and the fuel atomises well

 

At the other end of the speed range, assuming the same wide inlet choke above, airflow is relatively slow and the  fuel doesn't atomise well, and the result is low torque output. A smaller choke means faster airflow and better atomisation at lower engine speeds.

 

A double carb lets you manage both situations, giving a smaller single choke for low rpm's, high air intake speed with good atomisation = good torque...but when you open it up the limiting factor would be the small inlet choke diameter, so a second choke opens up to allow much more air through and as the revs continue to rise, a good high rpm power output.

 

Errr, just to be clear, when you describe the  'double carb' you are referring to a twin choke carb and not a dual/twin carb setup ?


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#12 fixitsan

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Posted 19 March 2019 - 05:15 pm

 

Errr, just to be clear, when you describe the  'double carb' you are referring to a twin choke carb and not a dual/twin carb setup ?

 

It could be either IMHO. The 2 choke throats could be in one carb (with simple manifold) or two carbs on one fancy manifold feeding a single or multiple cylinders. The important thing is that the two butterfly valves are not synchronised to open in parallel, but one opens after the other reaches about 75%. A solex twin choke with non-synchronous valves (one opens first then the second one opens), would be the same as two carbs on one manifold if both carbs are non sychronous too.

 

The only important thing here is that there are two indeependent intake tracts/valves, one for lower revs and the second one starts opening for higher revs while the first remains fully open.

 

I'm sure there are oddities falling between definiitions here and there. Generally though, small throat for faster airflow with better fuel mixing and cylinder filling at low speeds, ....and at the other end of the scale a very wide throat (or two throats  together) wide open with fast flow at high rpm's.


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