Flat slide carb question

[GS1000RACER]

Hi guys,

Can anyone tell me what differentiates flatslide carbs (like the VM29 smoothbores) from the standard carbs on the GS1000? I have a 1979 GS1000 and am considering swapping carbs. How are stock and smoothbore/flatslide carbs different. Thanks for the help!

[rcp]

VM29 smoothbores are actually roundslides rather than flatslides. The RS series are flatslides, VMs are roundslides - they're both smoothbores. Smoothbores are named such as pains have been taken to keep the bore as smooth as possiible - no obstructions, no divots to fit the slide. In the case of the roundslides, the slide itself has been hollowed out - effectively turning the bottom part into a thin-walled tube - and the walls of the bore have a thin arc machined in them through which the slide fits.

If that's clear as mud, I can post some pictures which would illustate.

- Richard

[posplayr]

Well

If that's clear as mud, I can post some pictures which would illustate.

I have several sets of CV's and CV Flatslide's and even a set of 40mm RS's as well as being an engineer and the splaination is less than instructive .

Pos

[sharpy]

This is what the bottom of 29/33 smooth-bores look like. well 33's have bigger nuts.

[rcp]

I have several sets of CV's and CV Flatslide's and even a set of 40mm RS's as well as being an engineer and the splaination is less than instructive .

(Here's where I try, and fail, to resist saying "Oh, an engineer! Ok, I'll speak slowly." :-) )

Actually It's less than instructive because it's not a very good explanation. There's a much better one in Kevin Cameron's Sportbike Performance Handbook, but I'll attempt to improve on it anyway.

There's several differences sort of jumbled up together here:
1) Smoothbore vs Solid-slide
2) Roundslide vs Flatslide
3) CV vs (dunno what to call it, so let's call it) "Direct-pull"

1) Smoothbore vs Solid-slide: In order to keep the airflow bore as smooth as possible, they designed hollow slides that could fit into thin slotted channels, thus minimally disrupting air-flow as it goes past the throttle slide channel. Solid-slide (non-smoothbore) had depressions cut out of the sides of the air-flow bore to accomodate the slide.

2) Roundslide vs Flatslide is pretty straightforward: earlier carburetors had a circular (cylindrical) bore for the throttle slide, and a cylindrical sliding throttle piston. Thus "Roundslide". Later carburetors (note: both CV and non-CV) had rectangular throttle slide bore, and correspondingly, a flat-faced rectangular slide. Thus "Flatslide". Part of the reason for this transition was to minimize the part-throttle-disrupting volume of air sitting under the slide (quite large in a hollow smoothbore round slide).

3) CV vs Direct-pull: On "direct-pull" carbs, the throttle cable is connected to the top of the slide, and pulls the throttle slide up mechanically. On Constant Velocity (CV) carbs, the throttle cable operates a butterfly behind the throttle slide, and that butterfly meters engine vacuum's access to a diaphragm connected to the top of the slide. When the CV throttle is opened, increased vacuum sucks the piston up at a rate that keeps airflow under it moving fast enough to pick up enough fuel. With non-CV carbs the rider has to control the throttle raise manually (smooth roll-on) otherwise not enough fuel gets picked up, (the air-fuel mixture leans out) and the engine stumbles.

Note smoothbore implies non-CV, so you'll often see them separated into smoothbore and CV carbs. Flatslides can be CV or non-CV, as can roundslides, however usually (not always) when they're referred to as such, they're talking about smoothbores.

- Richard

[rcp]

Illustrative Photos

Here's a VM33 Roundslide Smoothbore, note arced slits to accomodate the hollow cylindrical slide:


Here's an RS36 Flatslide Smoothbore, straight slits to accomodate the hollow rectangular slide:

[rcp]

As for which carbs you want, GS1000RACER, probably the first questions is, do you need to be period-correct or not? If you do need to be period correct, then you probably are limited to OEM CV and non-CV, and VM29, VM33 roundslide smoothbores and the Lectron flatslides.

- Richard

[pontiacstogo]

As for which carbs you want, GS1000RACER, probably the first questions is, do you need to be period-correct or not? If you do need to be period correct, then you probably are limited to OEM CV and non-CV, and VM29, VM33 roundslide smoothbores and the Lectron flatslides.

- Richard

Wouldn't 'period correct' also include Keihin CR's?

[Nessism]

Some nice photos there rcp. Thanks for going through the trouble.

I think the "smoothbore" term comes from the fact that there is an insert into the carb body (called a Jet Block) that improves air flow.

[rcp]

Wouldn't 'period correct' also include Keihin CR's?

Oops, yes those too.

- Richard

[posplayr]

Sorry

Richard,

(Here's where I try, and fail, to resist saying "Oh, an engineer! Ok, I'll speak slowly." :-) )

Sorry, I goaded u into that. I could have done better than the first attempt, but I was sure you could do better than me if u tried .

One last thing that might make your effort so far really enduring is to create a table of some common carbs and check them as

1) Smoothbore vs Solid-slide
2) Roundslide vs Flatslide
3) CV vs (dunno what to call it, so let's call it) "Direct-pull"

Stock GS Mikuni CV's (BS32SS, BS34SS, BS36SS)
1=Solid Slide; 2=round slide 3=CV

Stock GSXR Mikuni CV's Flatslide (BST34SS.BST36SS or BST38SS )
1=Smoothbore; 2=flat slide 3=CV

Mikuni RS (push pull)
1=Smoothbore; 2=flat slide 3=Direct-pull

I know this would be very helpful, I can even add what carb came on what years as I have many of the manuals. Above is about all I think I know .

Pos

TIA

[GS1000RACER]

As for which carbs you want, GS1000RACER, probably the first questions is, do you need to be period-correct or not? If you do need to be period correct, then you probably are limited to OEM CV and non-CV, and VM29, VM33 roundslide smoothbores and the Lectron flatslides.

- Richard

Hi Richard,

No, I'm not picky about being period correct. I would like some improved performance without going all out for some RS36's or the like. Sounds cheap, but I want a decent performance increase without paying much. Is it possible?

Tim

[Big T]

Hi Richard,

No, I'm not picky about being period correct. I would like some improved performance without going all out for some RS36's or the like. Sounds cheap, but I want a decent performance increase without paying much. Is it possible?

Tim

Well, 29 smoothbores would be the best match, but they're kinda collectors items, fetching $600-900 for a set

Some people use the 28mm pumpers that came on early Z1 Kawasakis. Otherwise, some 33s or Keihins are your best bet

[rcp]

Hi Richard,

No, I'm not picky about being period correct. I would like some improved performance without going all out for some RS36's or the like. Sounds cheap, but I want a decent performance increase without paying much. Is it possible?

Tim

Note that I'm far from a GS carb expert, but if you don't need period correct then you can get later CV carbs - from later model GSes, from GSXRs. The GSXR carbs are bigger bore, have flat slides, and should give you better part-throttle behaviour and easier rideability than the smoothbores would.

There's a GS1000G engine part-out as of today. The head and carb boots from that would give you bigger ports and the 40mm carb boots you'd need to mount VM33s smoothbores if you went that route. You'd likely be able to squeeze RS36 flatslides and GSXR 36mm CV carbs into those boots (I could make one RS36 fit into a single loose GS1000G boot I had), although the 36mm carbs seem to want 41mm or 42mm boots.

For bang-for-buck, the 36mm GSXR carbs probably win - they usually go for a lot cheaper than the VM29/VM33, CR33 and RS36 sets.

Whatever you get, don't forget to tune them: http://www.factorypro.com/tech/carbtun.html

- Richard

[rcp]

Oh, there's a pretty comprehensive list of GSXR carb models here: http://www.gixxer.com/forums/showthread.php?t=159718
I no longer own a set of the BST34SSes to double check, but I believe they'd fit the 40mm carb boots perfectly.

Oh and Pos, I believe any CV carb isn't a smoothbore, as it's got that big butterfly in the way. But I wasn't around when the term was coined, so can't say for sure what it means. Aside from that, all the BST model carbs are flatslide, CV.

- Richard