Roundslide carbs vs. Flatslide vs. D-slide (Keihin PWK)
(Originally posted on http://autos.groups.yahoo.com/group/vespasmallframes/message/4302)
All roundslide carbs suffer from the same design flaw. There is really no difference between the Mikuni roundslide and the Dell'Orto roundslide other than the position of the air correction screw. While on the Mikuni this adjustment is found before the slide, it is behind the slide on the Dell'Orto.
The problem in the design stems from the basic slide design. Its length and the shape below the slide promote turbulence which hinders proper airflow and atomization. The key problem is that (when looking at the front of the slide) the back part of slide is much lower than the front and the transition is not airstreamed. The incoming air hits the back of the slide and then bounces down toward the needle jet and what is not deflected down is spun into the inlet.
This problem is minimized on flatslides by making them much shorter; that generates a stronger pulse below the slide because the air flow is increased. On the D-slide [as on PWK] this problem is even further rectified by making the slide even shorter and the mid section of the slide is airflowed to minimize turbulence. All Keihin carbs (PJ, PWM, PWK) have flat and airflowed slides which is why there is less hesitation with them.