See this from Craig Weeks:
From: XS650 Performance <xs650performance@gmail.com>
Subject: power of flat side vs. round slide carbs
To: "Dennis" <dwyattjr@yahoo.com>
Date: Friday, January 23, 2009, 11:01 AM
A savvy XS650 builder recently did a dyno comparison between 34MM modern flat slide Mikunis (TM) and the older 34MM round slide (VM) versions. His engine is a 750 with 10.5 - 1 compression, a Jemco exhaust and a (he thinks) Shell-prepared head. He is running a Shell #1 cam.
On a Dynojet 200 the flat slides improved power 1 HP @ 4500 rpm and 1.5 HP at 7500. Max power with the flat slides was 64 @ 7500.
His jetting was 210 mains, 27.5 pilots @ 2.5 turns, no air jets, P-4 needle jets and 6F9 needles in the #3 position (one leaner than mid).
He claims throttle response is crisper than the round slides and while that is a seat-of-the-pants analysis, he describes the overall feel of the flat slides to be very good.
On the other side of the coin, he writes that the flat slides are way more difficult to fiddle with and more sensitive. One advantage is the flat slides are smaller and make a denser package.
He's very curious about how his new CNC head will work with the flat slides. So am I.
Thanks for the information, Tony.
Craig Weeks
I went this way and like the flat slides, but they are a bit more of a pain to service.