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Discussion Starter #1
With all the recent discussions about the pros & cons of various cylinder configurations and different displacements and carbs vs FI, it got me thinking.

In a twin engine motor, are two carbs better than one?
My kneejerk response is, well, each cylinder needs their own carb, right?
For a while I kinda thought Honda was cheaping out on the VTX1300 with only a single carb.

Then a buddy turned on the light bulb for me by mentioning that only one cylinder is firing at a time, so in practice, there is a carb for each cylinder.

Hmmm, then why do the manufacturers go to the extra expense of two or four or six carbs (depending on the engine)?

Plus, anyone with two or more carbs knows that occasionally they need to be sync'd to maximize efficiency.

One carb wouldn't require that step.

Although, now that I think about it, I've heard some people like to run the rear cylinder richer, since it is hotter.

Now I'm really confused......

Little help please?
-K
 

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Seems to me you covered most of the pros and cons.

True only one carb is needed since the fuel travels through an intake manifold. For best efficiency and tunablility a carb per hole per would make sense, but these bike are low HP low performance machines.

- My old 6cyl 200 Ford ran great on a single 1bbl carb. long way for the fuel to go though before reaching the 1 and 6 holes.

I am sure litnin will give us a very interesting synopsis of all the real reasons for wanting 1 vs multiple carbs. Hope he is around for this one anyway.
 

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A single carburetor is definitely easier to tune.

But, a large bore single carburetor cannot flow as much air (CFM)
as two smaller carburetors.

Providing both a single carb and dual carb setup is designed for the
engine it's being put on, the dual carbs will have more square area and
more capability of volumetric flow, than a single carb.

Therefore, the more air you can flow, the more fuel you can burn, the more
power you can make.

It's not always true that a dual carb will make more power, but in most cases, it is true.

Ability to fine tune is much greater with dual carbs as well.
As you said, you can tune each cylinder accordingly.

Also, dual carbs, since they flow better than single carbs, generally have
better performance higher in the RPM band, because they can breathe
better.

Single carbs usually perform better on the low end and provide better
low end torque curves. At high RPM, the single carbs run out of breath
where the duals can still breathe for a while longer, allowing them to
climb on up in the rpm range.
 

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Good info. I learned something then 8) So, my beef is still with Honda on the fact that since the 1100 has dual carbs and the breather is under the seat there is really no way to get the Spirit to "breathe" more. I'd think the 1100 could really spank if the design wasn't so restricting in that area.
 

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So that being said...
Why does a 4 cyclinder engine that comes stock with 4 single carburators (ie: CB 750 Honda) run better with (2) 2-into-1 adapters and two carbs instead of four singles? There is a website explaining this concept and actually sells the adapters. Click on the site:


http://www.cyclexchange.net/Honda Page.htm
 

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Discussion Starter #6
Wow, I'm getting an edumacation here for sure!
Thanks, I'll keep reading.

Interesting topic, I didn't know so much went into the lungs of our bikes!
-K
 

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I am currently waiting for a reply from that site to see if they have recommendations for the size of carb I need for my 750 Yamaha. The adapters can be modifiied to work on bike.
It's fun having other bikes to experiment with, just knowing there's another one sitting there, ready to ride!!
It's also a plus when you find a website where someone has already done all the work for you!!
 

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PainterD said:
So that being said...
Why does a 4 cyclinder engine that comes stock with 4 single carburators (ie: CB 750 Honda) run better with (2) 2-into-1 adapters and two carbs instead of four singles?
Consider the work they performed to achieve 15% hp increase:
Dyno testing was done for the proper fuel and air mixture.
Our test bike had 44+ thousand miles on it. Gave it a mini tune up, and we went from 51 HP to 58.8 HP.(rear wheel
-tuneup on the bike first (doesn't say what that included)
-removed the intake restrictions by adding velocity stacks
-rejetted the carbs using a dyno (the absolute best way to do this)
doesn't say if the exhaust was stock or not and I believe most models of the CB750 when introduced in the 60's started at 65+ hp.

That being said it's easy to see where the increase comes from and I doubt you could get an easier/better upgrade for the price.
 

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That's the best part about it...THEY did all the work for me. Looking at those 2-into-1 adapters, I could make my own for that price. With luck, I can make a pair of carbs I have laying around here work if they're the right size.
 

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PainterD said:
So that being said...
Why does a 4 cyclinder engine that comes stock with 4 single carburators (ie: CB 750 Honda) run better with (2) 2-into-1 adapters and two carbs instead of four singles? There is a website explaining this concept and actually sells the adapters. Click on the site:


http://www.cyclexchange.net/Honda Page.htm

What I mentioned was in general use and terms, primarily in stock apps.
It doesn't apply to performance modifications.
Yeah, it's very possible to make a single carb function better at
a given range, than a dual carb.

But with that being said, did you see the picture that looked down the
throttle bore?
That's not a stock sized carburetor. That thing is HUGE.
It's like looking down the barrel of mortar launcher.
There is a point when the square area of a single carb is going to be
bigger than that of the duals... but look at the physical size of the single
one compared to the individuals.

When Honda or any other engineers design vehicles, they design
for best performance, best fuel economy, and the amount of space
the thing has to work in. Those manifolds are taking up precious space
that might be used for something else.

Also, as Daver84 said, you don't know what else has been done.
I'd be willing to bet if you just honked that sucker on a stock motor,
it wouldn't run right. This is where these "performance parts" and "performance mods" get people snookered.
Yes, running performance parts can get you increases in power, BUT
when they advertise what they get, they'll tell you "this part gave us this result", when in fact, it was that part, plus this part, plus this part, plus
4 other parts.
 

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Then there is that whole V-Max setup that feeds each cylinder with two carbs when the rpm reaches 6500ish RPM....I believe that is what the V-boost does....apparently it hits hard when it happens like an old like a rochester 4bbl on a 350ci chevy.
 

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It was explained many years ago to me by a Royal Enfield/Honda dealer(sold Indians before that) when I asked to put the "dual-carbs" on my 750 Enfield which had a single.

"That single Amal carb is like a "carb and a half" and runs good for all that most riders need". The "dualies" are great for racing.....got a little more top speed, BUT for the street....they are a pain to "sync" and make these (Enfield) motors bog down to much...even when re-jetted. Never did go the dualie carbs on the Enfield.....it was fun sitting at the next light waiting for those "others" to catch up! :shock: :wink:

One or two moto mags did a feature article years ago about this very subject. And as always, "litnin" 'xplained the basics plus a lot more insight for those that still think changing to "dualies" will be worth the effort on a bike that came from the factory with a single carb.

Bullzeyet
 

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I guess my idea was to simplify things for tuning purposes, not so much to increase performance. Some things are better left up to the experts.
The manufacturers just do what they can with all the regulations they have to deal with when building these engines. It's funny some of them run at all the way they're tuned when you get them from the factory.
 

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PainterD said:
So that being said...
Why does a 4 cyclinder engine that comes stock with 4 single carburators (ie: CB 750 Honda) run better with (2) 2-into-1 adapters and two carbs instead of four singles? There is a website explaining this concept and actually sells the adapters. Click on the site:
http://www.cyclexchange.net/Honda Page.htm
cuse they just want to sell you a carb and an adaptor :wink:
 

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Not sure the adaptor will fit my bike. I would have to take some measurements off a 750 Honda and see what the difference is between that and my 750 Yammy.
**UPDATE**
They DID reply back to me (Yesterday) and said they would be doing further testing on other model engines in the future, so now I have to keep checking back with them once in awhile. >PD<
 
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