Flat Slide VS Round Slide Carbs



So I've seen this topic on a couple of other boards a few times, but most of them were just a bunch of listed opinions. It's not that I truly expect a difference when I post it here, but at least I can get the opinions of people I trust and if I can get any actual data or technical info in addition to that, then all's good.

Anyway... Can someone please cover the differences (mainly in performance and real-world applications) between these two types of carbs? I hear some say that flat slide are for racing use only or that round slides are smoother, but I'd like to get a better understanding as to whether that's even true and, if so, why?



Over 1,000 Posts
As Swivel mentioned, flats are usually much more responsive. The distance under the slide is considerably less than a round slide and the space for the slide is smaller, so there is less flow disruption too. That short slide creates a stronger signal "pressure drop" across the jets. The end result is tha they respond faster to a change in throttle and that makes the bike more responsive.

Some riders may find that to be annoying in traffic if they are used to automatic cars with non-responsive throttles. Most of us prefer that extra crispness.

They do flow better than the same nominal sized round slide carbs and make a touch more power at the top that a VM.


Makes sense... thanks guys.

Any opinions on how a set of VM28s (flat slide) would do on my 360?

Mods are are follows:
*+2mm over bore (378cc)
*+1mm over on valves (35mm intake, 29mm exhaust)
*3-angle grind
*Casting marks removed from ports
*+21° duration on the cam (251° total)
*+.040" lift (.381" total)
*Higher compression pistons (9.7:1, also considering decking head by a few more thou)
*Open intake (velocity stacks)
*Long two-into-two exhaust

Should be I looking at VM30s pretty soon instead of the VM28s?


Split personality, I fake being smart
They can also be really difficult to jet properly if your starting from scratch fitting them on something no one else has messed with.
Because of the flow rate, jetting can be completely wrong compared to round slide carbs of same size.
The flat/oval slide seems to work better for most people or even the GSX-R style flat/small semi-circle slides on CV carbs
VM 28's are actually plenty for a modified 360, I believe they will support about 30bhp per cylinder (I posted a list about a year ago but can't find it)

I think VM30's will support 35~40bhp per cylinder, your never going to make 70~80 bhp so either works (but 28's are easier to jet right)

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