fulltilt
XS650 New Member
Any riders/builders out there have experience with both flatslide and roundslide carbs? Any note worthy differences in temperment or performance?
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Flatslide vs. roundslide?
- Thread starter fulltilt
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- carburetors
dwyatt
XS650 Addict
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.
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.
Can't generalize about flat slides being "more difficult to fiddle with." Mikuni flat slide 4-stroke pumpers are much easier to dial in than VM's; better isolation of component effects make tuning almost a straight-line walk downstream. They're also the easiest carbs to service that I've ever owned; jet needles can be tuned without removing slides, needle jets thread onto the carb body and can, like the mains, be swapped out through the float bowl drain hole. The bell crank throttle makes synch a one-step process. Drawbacks are that they're not cheap, and unless you get the Topham Mikuni kit from Germany, a lot of fabrication is required.
zorpman
XS650 Member
Looked at the Topham Mikuni site. No prices listed. Any idea on the price of their flat side kit for the XS 650??
Last I heard it was around $600 US, shipped. Send them a message, they'll reply with current pricing.
fulltilt
XS650 New Member
Wow! What an amazing resource the 650 Forum is. Thanks for the really great info. Your thread has raised a few more questions however. Is the 34mm unit the optimum size for a motor modified as desribed? I have found many sources for mostly bigger models (36-40mm). Would they flow too much air? Another interesting thing is they all have an accelerator pump (or be pump compatible). Is a pump necessary? And last, a great many of the units I have found do not have a bell crank slide actuator. Is the bell crank assembly a have to have? Thanks again.
34 mm. is plenty of venturi, 36's also work well; performance differences are minimal. Unlike VM and cable-pull TM's, the RS (Radial Smoothbore) type pumpers are dedicated 4-stroke carburetors. The bell crank slide actuation is just part of that design, there's no magic to that; the advantages are more stable synch, only one synch adjustment to make rather than having to set both lift and drop, single throttle cable, ability to set idle speed without disturbing drop synch, and ease of access in making NJ position changes. The downside is that extended mounts can't be used, as clearance between throttle shaft and frame backbone is too close. TM and RS 4-stroke pumpers are a more recent and advanced design than VM's and cable-pull TM's.
650performance
XS650 Addict
Grizld1 is correct. 34mm is plenty for any XS 650 - 750 engine with hot street specs. Even on a 70 RWHP race engine there is only a fall off after 7500 rpm.
