I have been dyin to get my hands on one of these for a long time.  It may look like a regular Keihin PWK carb, but it’s a knockoff with an unusual brass contraption between the metering block and the main jet.  The Duderino has one, ThumperPaul has one, and The Sneeze has one.  I was anxious to learn more about it.

It’s a knockoff, and like all knockoffs there’s risk.  I might end up with a standard PWK like the ones I already have.  Curiosity overwhelmed me.  I really wanted to know exactly what’s inside one of these newer knockoffs with the unusual brass jet holder.  The minimal entry fee was worth the risk.  I ordered it, and in about four days the box was in front of my door.  That was a big improvement over the one-month wait time for the last knockoff.

There isn’t much difference when looking at the outside of the carb.  On the left side, you can see that the plastic idle stop screw has been replaced with a brass screw (nice upgrade), and you can see that the idle air screw looks different (it sticks out a bit and the head is larger).  Other than those two items, it looked the same as the other knockoffs I had purchased.

The upstream side had a few funky spots, but overall, it was much better than the previous knockoffs.

The slide needle is different.  The basic diameter is a lot smaller.  None of the available Keihin needles are close to this small.  If it needs a new needle, I am out of luck.  Here it is next to a genuine Keihin DGK (DGK on top, new needle on bottom).

Turns out the brass piece is an emulsion tube.  I’ll refer to it as the “E-Tube”.  Those little holes in the side of the tube alter the fuel delivery curve and admit air bubbles into the liquid gasoline.  The bubbles mix with the liquid and form an emulsion (i.e. foam).  The foam is lighter, so it responds faster to changes in vacuum, and promotes atomization.  That’s my limited understanding of the concept.

Put the straw in a glass of KoolAide (I like lime).  When you first start drinking, it doesn’t take much suction to get the KoolAide up the straw.

Venturi vacuum vs air flow is a quadratic function.  It is not linear.  As air flows through the venturi, the vacuum at the top of the needle jet increases exponentially.  So, when the air flow doubles the vacuum increases like four times.  That makes the fuel mixture go richer and richer as the rpm increases, but you want the air/fuel ratio to remain relatively constant (about 12.5:1).  To compensate, carburetors use air correction to try and maintain a reasonable mixture over a wide range of rpm.

The standard PWK and the Mikuni VM use a simple air correction system that does OK, but it fails to achieve a perfect air/fuel ratio over the entire range.  Both these carbs go rich at high rpm (when the throttle is wide open).  That’s why the aftermarket has come up with third fuel circuits like the “Thunderjet”, or the “Dial-A-Jet”, or the “InteliJet”, or the “Power Jet”.

You can put in a larger main airbleed to try and correct the rich condition at high rpm, but then you end up with a lean condition at lower rpm.  You can put in a smaller main jet to try and correct the rich condition at high rpm, but that will also result in going lean at lower rpm.  You can fiddle around with the airbleed and main jet until you go nuts, but you will probably never be completely satisfied.

This problem only pertains to WOT conditions.  At part throttle, the needle jet and slide needle are takin care of business.  But snap the throttle all the way open and those two components are out of the picture.  At WOT, you are relying on the main jet and air correction system to deliver fuel at the proper ratio.  That’s a tall order for a fixed jet with a fixed airbleed.

The emulsion tube offers a little bit of relief.  As the venturi vacuum increases exponentially, the fuel level in the emulsion well is dropping, and more emulsion holes are being uncovered.  The additional air correction helps to compensate for the exponentially higher vacuum.  It’s not perfect, but it should be an improvement over a simple airbleed system that only admits air at the very top of the needle jet.  With an emulsion tube, the naturally occurring drop in fuel level adds more air correction as the rpm rises.  It adds another dynamic component to the air correction system.

This YouTube vid does a nice job of explaining the quadratic behavior of a venturi.

https://www.youtube.com/watch?v=BvOqNwNuuAA

With the E-Tube and needle jet removed, you can see how the main airbleed passage ties into the emulsion cavity.  Just shine a flashlight up the hole in the metering block and look through the main airbleed.