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Yoshimura MJN 28 Jun 2021 19:55 #850866

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Interesting article but I have a couple of questions.

First, the needle jet isn't at the bottom of the venturi throat.  It's elevated slightly above it.  

Second, I would like to see a velocity profile across the venturi throat.  My guess is that flow is turbulent, which means the velocity would be pretty much the same across the throat except for very close to the walls.  

Third, there are two sources of energy inside the carb throat.  One is pressure and the other is velocity.  The sum of those two components equals the total energy in the throat, which remains constant at all times.  When one sourc increases the other must decrease because there is no other energy being added at any point between the intake and the outlet of the carb.  Mikuni carbs are designed with a wide intake that narrows down as air enters the throat.  The narrow throat forces air velocity to increase, and this occurs at the expense of pressure energy.  Air velocity increase means that pressure must DECREASE.  So pressure in the carb throat is the LOWEST of any point in the carb, not highest as stated.  This makes sense, because it's the difference in pressure between the throat and the bowl (at atmospheric pressure) that drives fuel up the main jet in the first place. 

I'm not saying that this technology isn't superior.  I don't really know.  It would be useful to see some atomizing experiments to demonstrate superior atomization.  And it would be useful to do some exhaust gas analysis that should show less unburnt fuel.  All I know is that the explanation in the article is not what I was taught in fluid mechanics.

Most people don't realize that needles and needle jets wear over time from sliding contact between each other.  After changing them out in both my Z1's I saw a 10% improvement in mpg, which I attributed to improved atomization in the throat.  

Yes, it is interesting. I can see how that would atomize fuel better than just the old school emulsion tube does.
On what carbs? And what parts did you use to replace said worn parts with? 

Someone once told me to marry that motorcycle I was riding ......there's times I wish I hadda listened .

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Yoshimura MJN 29 Jun 2021 03:46 #850873

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The carbs are Mikuni VM28 SC's.  They are standard Z1B carbs.  I only use Keyster rebuild kits.
I have a couple of Z1B's restored, an '80 KZ1000LTD restored, a 1981 KZ550 restored and a 2008 KLR 650 for off road fun. My wife has a 2019 Suzuki DR 650 for on and off road.

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Yoshimura MJN 24 Jul 2021 08:08 #852427

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So after a few hundred miles, everything is fantastic.
I'm getting about 35mph cruising at 75-80mph.
I still need to get on the dyno for good AFR readings and to perfect my timing.

The biggest issue I ran into was leaking Float Valve O-rings causing the carbs to overflow.  I need to keep a bag of them on hand because those seem to be the things that wear out first on the Keihin CRs (as pointed out by another forum member here a few years ago).

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