Dual Carb vs Quad Carb

[skyshadow]

Before you say something, I used the search button with great intent but found nothing.
I'm curious of the pros and cons of a dual carb kits (such as the one sold on Speedmotoco.com) compared to quad carb kits (such as Keihin). Anyone around here have more knowledge about air flow and "shit" than I?

 

[cxman]

talk to alpha dog he has installed some of my kits and recently installed tuned and drove one of the cb750 dohc kits

 

[teazer]

I know shit about airflow, does that help

Old Brit bikes used single carbs on twins and twin carbs on thr more sporty models but that didn't make a huge difference. So it looks like that will work BUT a Brit twin is 360 degree and that means each time the pistons go down only 1 cylinder is breathing in. So they alternate left and right and everything is fine.

On an inline 4 the two outer and inner pairs are like that, so if 1-4 and 2-3 were connected that way all would be fine, but that's not practical. If we connect 1-2 and 3-4 there's a period where intake pulses tend to get in each others way. Obviously then a 1-2 3-4 won't work as well, but the real question is does it work well enough for the street?

The answer seems to be that with larger carbs, jetted by someone that knows that particular bike and has access to a dyno and exhaust gas analysis they can work well enough. Wrong jets and it could be a PIA to get right.

And that brings us to the obvious question of WHY? Stock carbs with stock airboxes work fine. Some stock slide carbs work OK with individual filters. Stock CV carbs and pods are not a good answer in most situations.

And CR carbs are awesome but not cheap and not necessary for most of us.

And for the record, I go drag racing on my 2 stroke with CV carbs and pods which we all know cannot work, but it does. So to get back to your question - what bike and what's your thinking? It's best to define the problem before you chase the solution that may or may not be a good fit.

 

[skyshadow]

Wow, lots of info there. The cylinder stokes makes a lot of sense.
As for why, Because I'm doing a CB900 rebuild, either going with Kehien Carbs or some duals. I just saw the duals were a little cheaper, so I became curious.
So what I'm getting is duals will not capitalize on power potential?

 

[cxman]

if you are an all out racer you might do better with quad carbs

my personal cb900 custom has twin 36mm flat slide mikunis on it and it runs like a raped ape

in the 2 years i have had them on it i have not had to fiddle with them once since the install

 

[AlphaDogChoppers]

My experience with murrayscarbs.com has been stellar. I recently did a set on a CB750f, and the bike pulls like a freight train.

Using dual carbs has efficiency benefits much like you get with a 4-into-1 exhaust. The flow through each carb is more continuous than when each carb must wait for the next intake impulse of the cylinder it is dedicated to. The powerband comes on MUCH sooner with the dual carbs than with 4 carbs. In essence, when an intake valve opens and is ready to draw fuel/air mixture, the charge from the carburetor is already moving because another cylinder was just drawing from that carburetor. That is a bit of an oversimplification that doesn't really explain exploitation of the pressure pulses that herd the fuel/air charge into the cylinders in a bigger hurry, but it gives you an idea of the premise of why this works.

I've never been too fond of the Keihin carbs that come on these bikes. I do know that they work a lot better on the street if you keep the airbox. I always modify airboxes for better airflow, and install the K&N filter, and rejet. That's OK if you have a good working set of carbs. If your carbs are screwed up or missing, the Murray's Carbs set up is a definite upgrade, not so much for flat-out top end, but a lot more grunt and satisfying throttle response during typical riding.

If your build is going for the "look" with pod filters, the Keihin carbs are a poor choice. CV carbs can be made to work OK with pods, but not their best. Slide carbs, like Murray (CXMAN) uses thrive on pod filters.